UC-NRLF 


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AEEAL   GEOLOGY  OF  0?HS   CREEDE 
DISTRICT,    COLORADO. 

By 


X     ' 
ESEBR  S*   LARSEIT. 


H  B  S    IS 


Submitted  in  partial  fulfillment  of  the  require 
ments  for  the   degree  of  Doctor  of  Philosophy  in 
the  University  of  California. 
May,   1918. 


Approved: 


The  Areal  Geology  of 


The  Greede  Mining  District, 


Esper  3.  Larson. 


COKTEIIT3 . 

Chapter  I.        Introduction 1 

Location  and  topography 1 

Field  work  and  acknowledgments 

History 

Production     ......    10 

Dining  and  treatment  of  ores 11 

General  conditions  • 11 

Humphreys  mill 13 

Amethyst  mill 15 

Solomon  mill 16 

Hidge  mill 16 

Bibliography 18 

Chapter  II.  Preliminary  outline  of  the  geology 21 

Age  of  the  rocks 21 

The  Potosi  volcanic  series  22 

Lower  division 

Upper  division   • 

The  Creede  formation  26 

The  llacXenzie  .'-.fountain  quartz  latite 26 

Intrusive  rocks  27 

Quaternary  deposits 27 

Geologic  structure 28 


782075 


Chapter  III.  Bocks  of  the  lower  division  of  the  Potosi 
volcanic  series .••• 


General  statement • • 

Outlet.  1?unnel  quartz  latite 

General  character  and  occurrence  

Petrography 

Megascopic 

Microscopic  .................... 

Less  coraon  rock  types   

V/eathering  and  outcrops • 

willow  Creek  rhyolite 

General  character  and  distribution  ........... 

Petrography .,. 

Megascopic ...• 

Microscopic  .  .  .  

Chemical  ......  .  

Structure 

",7eathering  and  alteration 

Topography  and  scenery   • 

Ohapter\IV.  Rocks  of  the  upp^r  devise  of  the  Potosi  volcanic  series 
GeneraX  statement 


29 
29 


1 


Hornblende- quartz  latite 

General  character  and Distribution 

Thi 

Petrography 

Megascopic 

-.iicroscop 


_  rhyolite. 
1 

General  character  and  distribution. 

Contacts. 

Petrography, 

Megascopic. 

Microscopic. 

feathering  and  outcrops. 
Distinction  from  other  rhyolltea. 
Phoenix  Park  quarts  latlte, 

General  character  and  distribution. 
Petrography. 

Megascopic. 

Microscopic, 


Outcrops  and  Weathering. 
Equity  quarts  1 at its. 

General  character  and  occurrence. 

Contacts, 

Petrography. 

Megascopic. 

Microscopic. 


Chapter  IV.  Rocks  of  the  upper  division  of  the  Potosi  volcanic  series 
General  statement 
Hornblende-quartz  latite 

General  character  and  distribution 

Thickness 

Petrography 

Megascopic 
Microscopic 

Weathering  and  outcrops 
Windy  gulch  rhyolite  breccia 

General  character  and  distribution 
Petrography 

Megascopic 
Microscopic 

Weathering  and  outcrops 
Comparison  with  Campbell  Mountain  rhyolite 
Hazanoth  Mountain  rhyolite 

General  character  and  distribution 
Character  of  contacts  and  thickness 
Petrography 

Megascopic 
Microscopic 

Weathering  and  outcrops 

Comparison  with  the  Campbell  Mountain  rhyolite 
Hhyolite  tuff 

General  character  and  distribution 
Petrography 


Tridyadte  latite. 

General  character  and  occurrence. 

Thickness* 

Petrographjr, 

Megascopic. 

Microscopic. 

Chemical. 

The  rocks  n*ar  the  aouth  of  Beer  Hom  Creek* 
Outcrops,  Weathering,  and  alteration. 
Andes it e, 

General  character  and  distribution. 

i 
Petrography, 

General. 

Microscopic. 

Alteration  and  topography. 
Quarts  latitic  tuff, 

General  character  and  occurrence. 

Thickness, 

Petrography. 

Massive  rock  in  the  tuff. 

Weathering  and  outcropa. 
Lks 
quart  •  latlte. 

General  character  and  distribution. 

Thickness* 

Petrography* 

Megascopic. 

Microscopic, 
Weathering,  outcrops,  and  topography* 


6 

Nelson  Mountain  quartz  latite 

General  character  and  distribution 

Thickness 

Petrography 

Megascopic 
Microscopic 

Weathering  and  outcrops 
Chapter  V.  Creede  Formation 

Name,  general  nature,  and  occurrence 
Structure  and  thickness 
Subdivision 
Lower  member 

General  character 

Petrography 
Traverture 

Character  and  distribution 

Origin 
Upper  member 

General  character  and  distribution 

Petrography 

Massive  rook 

Weathering  and  topography 
Origin 

Position  in  the  section 
Age 


Chapter  VI.     Latest  series  of  Lava  Plows 
General  statement 
MacKenzie  Mountain  quartz  latite 

General  character  and  distrilmtion 
Character  and  thictaiess 
Petrography 

Megascopic 
Microscopic 

Weathering  and  outcrops 
Chapter  VII.  Intrusive  rocks 
General  statement 
Rhyolite  porphyry 

General  character  and  distribution 
Relation  to  adjoining  rocks 
Petrography 

Megascopic 
Microscopic 


Quarts  latite  porphyry. 
Occurrence* 
Age, 
Petrography. 

Megascopic, 
Microscopic. 
Intrusive  basalt. 
Occurrence, 
Petrography. 

Chapter  VIII.       Quaternary  deposits. 
Introductory  atatement. 
Moraines. 

Distribution. 

Glacial  to^ographju 

Two  stages  of  glaeiation. 

Earlier  moraine. 

Later  moraine* 

Character  of  material. 

Poet  glacial  erosion* 

Economic  considerations. 
Terrace  gravels* 
Landslides. 

denftral  statement. 

General  distribution, 

Distribution. 

Economic  consideration. 


Talus. 

Alluvial  faun 3.3. 
Alluvium. 

Chapter  II,       Structure, 
Introduction. 
Faulting. 

Difficulty  of  detecting  faAltg. 

Gtoneral  «haraat<»r  of  the  faulting. 

Age. 

Mineralization  along  faults. 
The  Amethyst  fault  eyttenu 


/  V  ^ 

v"  •  A/ 


—T — T-         ~ re  Mine 

^.iivi'i.iijXi  poiti^, 
Equity  Fault.  ) 

Fault  block  of  Mammoth  Mountain. 
Bulldog  Mountain  fault, 
Ridge  fault. 
Alfaha  fault. 

Structure  and  faulting  northeast  of  Sunny side. 
Minor  faults* 


II 

t 


Geology  and  ore  deposits  of  Creede,  Colorado, 
W.  H.  Emmons  and  3.  S.  Lars  en. 


Introduction, 

Location  and  topography.-  The  Creede  mining  district  is  in  Mineral 
County,  southwestern  Colorado,  near  the  eastern  border  of  the  elevated 
region  which  is  generally  known  as  the  San  Juan  Mountains.   The  town  of 
Creede  is  situated  on  77illow  Creel-i,  a  few  miles  above  its  junction  with 
the  Rio  Grande.  The  area  included  within  the  borders  of  the  Creede 
special  map  is  about  4g-  miles  east-west  and  5J-  miles  north-south. 

3C3ZCCEEX&I 


mi  h  MM  in  I  1 1   The  lowest  portion  of  this  area,  about  one  mile  below 
Creede,  is  about  8,700  feet  above  sea  level,  and  the  highest  is  the  summit 
of  ITelson  Mountain  near  the  north  border  of  the  area  which  is  12,029  feet 
above  sea  level.  The  relief  is,  therefore,  over  3300  feet.  Only  a  few 
miles  to  the  north  is  San  Luis  Peak,  which  is  over  fourteen  thousand  feet 

in  elevation. 

The  area  may  be  divided  into  four  topographic  units-,  the  valley  of 
the  Rio  Grande  with  its  gentle,  rolling,  grass  covered  hills;  the  sharp, 
rugged  canyons  of  the  main  streams;  the  gentle  slopes  in  the  upper  drain- 
age; and  the  rounded  slopes  and  mesas  of  the  uplands.   The  valley  of 
the  Rio  Grande,  which  is  cut  in  the  soft  lake  beds,  is  about  two  miles 
wide  near  Creede  which  is  on  the  north  border.-   It  is  shown  on  Plate 
about  Creede  with  the  rolling  hills  of  lake  bed  tuff,  and  the  alluvial 

flat  of  the  streams. 


The  canyons,   especially  that   of  Billow  Creelc  above  Creede,  present   sheer 
cliffs  several  hundred  feet  high,  nearly  vertical  and  locally  overhanging. 
Near  the  Monte  Carlo  nine   or  a  thousand  feet   vertically,   the  cliffs  have  an 

tt 

average  slope  of  about  sixty  degrees.  Plate  ^opposite  p.   ,  shows  these 
cliffs  above  Greede. 


-&^^ 


In  the  Tapper  parts  of  the  streams  recent  glaciation  has  modified  the 
valleys  and  given  gentler  slopes 

o^East  " 


^bg»j&?cvu&s£r9^&&>^^  dV 

Lise  most  of  the  mountainous  area  of  the  San  Juan,  the  region  is  tim- 
bered and  well  watered.   It  is  served  by  a  broad  guage  branch  of  the  Denver 
and  Rio  (Jrande  railway,  which  daily  carries  sleeping  cars  from  Denver  and 
Pueblo.  The  district  is  more  accessible  than  many  camps  of  the  San  Juan 
and  rates  for  shipping  ore  are  considerably  lower. 


A3 


Bibliography. 

On  the  geology  and  ore  deposits  of  the  San  Juan  there  is  an  extensive 
bibliography.   Lists  of  the  publications  of  the  survey  may  be  found  in 
United  States  Zoological  Survey  Bulletin  ITo.  507,  by  J.  M.  Hill.   Below  are 
listed  only  those  papers  treating  the  geology  and  ore  deposits  of  the  Creede 
district  which  is  separated  from  other  areas  of  mineralization  in  this 
region  by  a  fairly  extensive  area  that  is  not  Imown  to  contain  mineral 
deposits.  Besides  the  papers  listed  there  are  also  numerous  brief  notes 
of  timely  interest  in  the  technical  press  and  mining  journals. 
iliiiiifci 
Emmons,  7.  H. ,  and  Larsen,  E.  S.,  A  preliminary  report  on  the  geology  and 

ore  deposits  of  Creede,  Colo.,  in  Bull.  U.  S.  Geol.  Survey  no.  530, 

1913,  pp.  42-65. 

Discusses  briefly  the  principal  results  of  the  United  States  G-eological 

Survey  investigations  at  Creede,  of  which  the  present  paper  is  the 

final  report. 

Emmons,  ',7.  H.,  and  Larsen,  E.  S.,  The  hot  springs  and  mineral  deposits  of 
•tfagon  'Theel  (Jap.  Econ.  Geol.,  vol.  8,  1913,  pp.  235-247. 
Describes  a  fissure  vein  near  which  hot  springs  now  issue. 

Henderson,  Charles  W. ,  Colorado.  Gold,  silver,  copper,  lead,  and  zinc  in 

western  states:  Min.  Res.  U.  S.,  1908,  Pt.  1,  Metallic  products,  p.  390, 
Table  of  production  and  notes  on  development  at  Creede. 


Henderson,  Charles  ",7. ,  G-old,  silver,  copper,  lead,  and  zinc  in  the  western 
states,  Colorado.  Min.  Res.  of  the  U.  3.,  1909,  Ft.  1,  p.  320. 

Tables  of  production  and  notes  on  the  mining  conditions  at  Creede. 
Seven  companies  produced  ore,  the  total  tonnage  of  Mineral  County  being 
64,941  tons.  Over  one  half  of  the  production  was  milled,  the  remainder 
being  shipped  to  smelters  as  crude  ore. 

Henderson,  Charles  ,7. ,  Gold,  silver,  copper,  lead,  and  zinc  in  the  western 

states  by  H.  D.  MsCasloey:  Min.   Res.   of  the  U.   S.,   1910,  Pt.   1,  Metals,  p.425, 

^ 

The  statistics  of  production  in  1910  and  mining  conditions  of  the 

district  are  reviewed. 

Henderson,  Charles,  tf. ,  Colorado,  in  gold,  silver,  copper,  lead,  and  zinc  in 
the  western  states  by  H.  D.  SfcCaslcey:  Min.  &es.  of  the  U.  S.,  1911, 
Pt.  1,  Metals,  p.  551. 

Lars en,  E.  3.,  See  Smmons  and  Larsen. 

Larsen,  E.  S.,  and  ",7ells,  R.  C.,  Some:  minerals  from  the  fluorite-barite  vein 
near  7agon  Wheel  Gap,  Colo.,  proo.  Nat.  Acad.  So.  2,  360,  1916. 

Larsen,  E.  S.,  and  'tfherry,  E.  T.,  Halloysite  from  Colorado,  Sour.  Wash.  Acad. 
Sc.  VII,  178-80,  1917. 

Lee,  Harry  K. ,  Gases  in  metalliferous  mines:  Proc.  Colo.  Scientific  Soc. , 
vol.  7,  1903,  pp.  163-188. 

Discusses  the  geology  and  ore  deposits  of  the  Amethyst  Lode,  and 
described  the  gas  that  issues  from  the  walls  into  workings  in  the  north 

end  of  the  Amethyst  Lode.  An  analysis  of  the  gas  is  reported. 


AT" 


Lindgren,  'tfaldemar,  and  others.  Gold  and  silver  in  Colorado,  in  Min.  Res. 
of  the  U.  S.,  1905,  p.  205. 

Gives  a  table  showing  production  of  Mineral  County  and  notes 
on  mining  conditions  at  Creede. 

ITaramore,  Chester,  Colorado  in  gold,  silver,  copper,  lead,  and  zinc  in 
western  states:  Min.  Res.  of  the  U.  S.,  1907,  Pt.  1,  Metallic 
products,  p.  262. 

Notes  on  developments  at  Creede,  with  a  table  showing  production. 

Parmalee,  H.  C.,  Zinc  ore  dressing  in  Colorado;  the  Creede  district: 
Metall.  and  Chem.  Eng.  ,-+&*  .  ^  my>  Dec.  1910,  pp.  677-678. 

Describes  the  treatment  of  Creede  ores  at  the  concentrating 
plants  of  Greede. 

Richard,  T.  A.,  The  development  of  Colorado's  mining  industry:   Trans.  Am. 
Inst.  Llin.  Sng. ,  vol.  16,  1896,  pp.  834-848. 

G^ves  an  account  of  the  early  history  of  the  Creede  district  and 
of  its  early  production. 


Chapter  II, 
preliminary  outline  of  the  geology. 

The  "bed  rock  exposed  in  the  area  included  in  the  Creede  and  Vicinity  quad- 
rangle and  for  a  number  of  miles  in  all  directions  is  made  up  entirely  of  Ter- 
tiary volcanic  rocks  and  is  a  part  of  the  great  volcanic  field  of  the  San  Juan 
Mountains.  Lava  flows  make  up  the  greater  part  of  the  material  "but  tuff  and 
breccia  deposits  are  important  and  a  few  small  intrusive  "bodies  are  present. 
Although  the  rocks  show  little  variety  and,  with  the  exception  of  a  single 
andesite  formation,  are  all  classified  as  rhyolites,  rhyolite  "breccias,  and 
quarts  latites,  they  "belong  to  four  distinct  periods  of  eruption  and  are 
separated  by  very  irregular  surfaces  of  erosion,  the  rocks  of  each  eruptive 
series  consisting  of  a  number  of  subdivisions  which  are  themselves  commonly 
separated  by  irregular  surfaces  of  erosion. 

In  the  course  of  the  general  survey  of  the  San  Juan  Mountains,  which 
has  been  in  progress  for  some  years,  under  the  direction  of  "^hitman  Cross, 
the  volcanic  rocks  have  been  studied  and  mapped  over  nearly  the  whole  of 
the  mountains  including  the  San  Cristobal  and  the  Creede  quadrangles.  The 
lavas  and  associated  lake  beds  and  other  clastic  deposits  of  the  Creede 
area,  while  in  part  local  in  extent  and  character,  are  a  normal  part  of  the 
large  San  Juan  sequence  and  the  two  lower  groups  of  eruptive  rocks,  or  those 
underlying  the  lake  beds,  correspond  to  subdivisions  of  the  potosi  volcanic 
series.  The  lower  part  of  the  lower  group,  including  the  Outlet  Tunnel 
quartz  latite,  the  Willow  Creek  rhyolite,  and  the  Campbell  Stountain  rhyolite, 
differs  somewhat  from  the  normal  rocks  at  this  horizon,  and  as  these  rocks  are  not 
knovm  at  any  great  distance  from  Creede,  they  orobably  represent  a  group  of 
local  flows. 


/7 


Age  of  the  rocks. 

The  rocks  are  all  believed  to  "be  of  Miocene  age  as  plant  remains  collected 
under  Bristol  Head  to  the  west  from  a  horizon  between  the  lower  and  upper  div- 
isions of  the  Potosi  volcanic  series  are  closely  related  to  those  of  the  Flor- 
issant lake  beds  w.iich  are  Miocene  and  probably  upper  Miocene  in  age,  while 
plant  remains  collected  from  the  overlying  Creede  formation  are  also  closely 
related  to  the  Florissant  flora. 

Potosi  volcanic  series. 

Lower  division. 

The  oldest  rocks  exposed  in  the  quadrangle  comprise  a  succession  of 
rhyolites  and  quartz  latites,  chiefly  in  flows  but  containing  some  clastic 
material,  forming  the  lower  division  of  the  Potosi  volcanic  series.   The 
lowest  formation  of  this  series,  the  Outlet  Tunnel  quartz  latite,  is  exposed 
only  in  two  small  areas  in  the  bed  of  East  Willow  Creek.   (See  plate  II). 
It  is  made  up  chiefly  of  hornblende  quartz  latite,  but  contains  some 
pumiceeus  rhyolite;  It  contains  both  flows  and  fragmental  material.   It  is 
overlain  irregular Itt- by  the  v/illow  Greek  rhyolite  which  is  mdde  up  of  sev- 
eral flows  of  purple-drab  to  gray,  fluidal,  banded,  felsitic  rhyolite. 
Just  north  of  Creede  over  anthousand  feet  of  this  formation  are  exposed 
.vi^yout  evidenceof  more  than  a  single  flow.   The  Campbell  Mountain  fhyolite 
overlies  a  somewhat  irregular  surface  of  the  pillow  Creek  rhyolite  and  is 
made  up  of  flows  of  a  dull  reddish  brown  or  mottled  flow  breccia.  It  attains 
a  thickness  of  a  thousand  feet  near  Creede  but  becomes  much  thinner  to  the 
northeast  a.-.d  east.   The  phoenix  Park  quartz  latite  is  chiefly  over  the 


Campbell  Mountain  rhyolite  but   is  in  part   interbedded  with  it.     The  Phoenix 
Park  quartz  latite   is  commonly  a  light  red-brown  biotite-hornblende-quartz 
latite,  chiefly  in  flows  but  containing  some  rather  coarse  breccia.      It 
attains  a  great  thickness  at   the  head  of  2ast  *7illow  Creek  but   in  the 
Creede  and  Yicinity  quadrangle  only  a  few  hundred  feet   are  exposed.     The 
Equity  quartz  latite   is  closely  related  to  the  phoenix  Park  quartz  latite 
and  probably    represents  a  great   flow  in  that   series.      It  was  recognized 
only  in  the  iiorthern  portion  of  the  quadrangle  in  the  drainage  of  \7est 
'7illow  Creek  where  it  overlies  the  Campbell  Mountain  rhyolite  rather 
regularly  and  has  a  thickness   of  about   a  thousand  feet.      It  is   in  large 
part   a  single  flow  of  a  quaker-drab,   fluidal,  biotite-quartz  latite. 

Upper  division, 

The  rocks  of  the  upper  division  of  the  Potosi  volcanic  series  have 
at   their  base  a  surface  of  great  irregularity  and  their  extrusion  was 
evidently  preceded  by  a  considerable  period  of  erosion  during  which  the 
streams  cut  canyons  in  the  rock  of  the  lower  division  comparable  in  depth 
and  ruggedness  to  those  of  the  present   stream,  but  in  no  wise  related  to  the 
canyons  of  the  present  day.       The  rocks  of  the  upper  division  filled  in 
these  new  made  canyons  aixl  probably  covered  nearly  or  quite  all  of  the 
region.     The  lowest   of  this  series  of  flows  is  a  hornblende-quartz  latite  in 
thin  flows  and  breccia  beds  and  is  exposed  only  in  the  drainage  of  Rat   and 
Lliners  creeks.     Locally  it   is   several  hundred  feet   thick,   else^ere  it   is 
entirely  absent.      It   is   overlain  by  a  few  hundred  feet   of  light  red-brown 


rhyolite  flow  breccia  and  tuff  (the  Tindy  Gulch  rhyolite  breccia),  charac- 
terized by  abundant  included  fragments  of  pumice.   A  great  flow,  locally 
four  hundred  feet  thick,  of  tridymite  latite  overlies  rather  irregularly 
this  rhyolite  breccia.  The  rock  is  red-brown  in  color  and  is  character- 
ized by  prominent  fluidal  banding,  by  the  presence  of  much  tridymite  in  the 
more  porous  bands,  and  by  abundant  millimeter-sized  crystals  of  orthoclase, 
plagioclase,  and  biotite.  The  tridymite  latite  is  overlain  rather  regularly 
by  a  series  of  andesites  which  vary  greatly  in  thickness  but  nowhere  exceed 
five  hundred  feet. 

In  the  eastern  part  of  the  quadrangle  the  two  lower  formations  of  the 
upper  division  of  the  Potosi,  described  in  the  preceding  paragraph,  are 
wanting  and  their  place  is  taken  by  a  great  flow  of  rhyolite,  the  Mammoth 
Mountain  rhyolite  which  is  locally  a  thousand  feet  thick.  This  rock  is  a 
red-brown  flow  breccia  and  is  very  similar  to  the  Campbell  Mountain  rhy- 
olite of  the  lower  division.   It  is  overlain  by  several  hundred  feet  of 
rhyolite  tuff  with  associated  thin  flows  and  this  is  in  turn  overlain  by 
the  tridymite  latite  to  the  east  of  the  quadrangle. 

A  succession  of  quartz  latites  overlies  rather  irregularly  the  ande- 
site  or  tridymite  latite  and  locally  rests  On  other  rocks.   These  quartz 
latites  carry  rather  abundant  millimeter-sized  crystals  of  plagioclase, 
quartz,  orthoclase,  biotite,  augite,  hornblende,  and  titanite.   The 
lowest  formation  of  this  succession  consists  of  several  hundred  feet  of 
nearly  white  quartz  latite  tuff  with  some  interbedded  flows.  The  inter- 
mediate formation,  the  Hat  Greet-  quartz  latite,  is  made  up  in  large  part 
of  flows  but  contains  some  tuff.  The  upper  formation,  the  Nelson  Mountain 
quartz  latite,  is  a  persistent  mesa-forming  flow,  about  two  hundred  feet  thick. 


The  Greede  formation. 

The  Oreede  formation  was  deposited  in  a  lake  which  occupied  a  valley 
carved  out  of  the  rocks  of  the  potosi  volcanic  series.  This  valley  was 
deeper  than  that  of  the  present  valley  of  the  Rio  Grande  and  occupied  about 
the  same  position  from  Antelope  Park  to  'Tagon  ".'/heel  0-ap. 

The  lower  part  of  the  Greede  is  male  up  chiefly  of  fine  textured,  thin 
bedded  rhyolite  tuffs  with  some  coarser  material,  especially  near  the  borders 
of  the  old  lake.   It  contains  numerous  bodies  of  travertine  which  indicate  the 
presence  of  abundant  hot  springs  during  this  period.  The  upper  part  of  the 
Greede  formation  is  somewhat  coarser  textured  and  consists  chiefly  of  fairly 
well  bedded  breccia  and  conglomerate  with  some  fine  tuff  and  intercalated 
thin  flows  of  soda  rhyolite. 

MacKenzie  Mountain  quartz  latite. 

A  later  series  of  lava  flows  overlies  the  older  rocks  very  irregularly 
and  in  the  area  included  in  this  report  is  made  up  of  a  single  great  flow, 
the  MacKenzie  Mountain  quartz  latite,  which  is  characterized  by  abundant 
large  crystals  of  plagioclase,  biotite,  and  augite.  To  the  north  and  east 
this  series  embraces  a  great  thickness  of  flows  and  tuff  breccia  deposits 
made  up  of  quartz  latites  and  related  rocks  which  are  characterized  by 
abundant  large  phenocrysts. 

Intrusive  rocks. 

Intrusive  rocks  are  present  only  as  a  few  comparatively  small  bodies. 
Three  types  have  been  recognized.  The  oldest  is  a  rhyolite  porphyry  which 
was  intruded  as  irregular  or  sill-like  bodies  into  the  rocks  of  the  lower 


division  of  the  Potosi  volcanic  series.   It  is  a  nearly  white  rock,  char- 
acterized by  large,  glassy  orthoclase  crystals.   A  single  small  dike  of 
andesite  cuts  the  rocks  of  the  upper  division  of  the  Potosi  volcanic  series 
west  of  Rat  Cree^.   In  'liners  Creek  a  number  of  dikes  of  quartz  latite  por- 
phyry tfut  the  rocks  of  the  upper  division  of  the  Potosi  and  are  probably 
related  to  the  flows  of  the  MacKenzie  Mountain  quartz  latite.   The  rock 
of  these  dikes  is  dense  and  carries  rather  abundant  large  crystals  of 
plagioclase  and  some  of  biotite,  augite,  and  hornblende. 

Quaternary  deposits. 

Following  the  volcanic  activity,  erosion  again  became  the  dominant 
geologic  agent  and  the  present  mountains  and  canyons  were  carved  from  the 
great  volcanic  pile.  After  the  streams  had  cut  down  their  beds  to  nearly 
their  present  level,  the  upper  parts  of  the  main  streams  were  occupied  by 
glaciers  which  widened  the  valleys  but  did  not  cut  them  much  deeper. 
Since  the  disappearance  of  the  glacial  ice  the  streams  have  deepened  their 
channels  but  little. 

The  terminal  moraines  of  the  glaciers  of  Rat  Creek^and  both  forks  of 
",7illow  Creek  are  within  the  area  included  in  this  report.  Large  landslides 
are  important  geologic  features,  and  talus,  terrace  gravels,  and  alluvium 

cover  small  areas. 

Geologic  structure. 

The  c;iief  structural  feature  of  the  region  is  a  rather  complex  block 
faulting  with  some  tilting  near  the  faults.  The  ore  deposits  are  along  these 
faults.  The  faults  are  believed  to  be  later  than  any  of  the  volcanic  rocks 
of  the  area  and  they  are  earlier  than  the  development  of  the  present  topography. 

In  addition  to  the  faulting  there  is  probably  a  gentle  tilting  toward  the  south. 


Chapter  III. 
Rocks  of  the  Lower  Division  of  the  Potosi  Volcanic  Series. 

G-eneral  statement. 

The  oldest  rocks  exposed  within  the  area  covered  by  the  Creede  and  Vic- 
inity map  are  a  thick  series  of  rhyolites  and  quartz  latites  forming  the 
lower  division  of  the  Potosi  volcanic  series.  Only  a  few  miles  to  the  west 
under  Bristol  Head  they  are  underlain  irregularly  by  a  series  of  andesitic 
rocks  and  still  farther  up  the  Bio  Grande  near  the  mouth  of  Lost  Trail  Greek 

i/ 

they  overlie  the  San  Juan  tuff.   In  other  places,  as  in  Upper  Ute  Creek  to 
the  west  of  Creede  and  in  Cebolla  Creek  to  the  north  of  Creede,  they  or 

\J  Cross,  'Thitman,  U.  3.  3eol.  Survey  Seol.  Atlas,  Silverton  folio 
(No.  120),  p.  7,  1905. 

younger  volcanic  rocks  directly  overlie  pre-Cambrian  granites,  schists, 
gneisses,  quart zites,  and  other  rocks;  in  others  still,  as  near  Pagosa  Springs 
and  south  of  the  Gunnison  River,  they  overlie  Paleozoic  and  Liesozoic  sediments. 

The  top  of  this  division  is  likewise  a  surface  of  marked  irregularity,  due 
to  the  considerable  period  of  erosion  which  immediately  preceded  the  extrusion 
of  the  rocks  of  the  overlying  upper  division  of  the  Potosi  volcanic  series. 
A  few  miles  to  the  west  a  great  thickness  of  andesitic  rocks  lies  between  these 
two  divisions  and  beneath  the  erosional  surface.   In  the  Creede  Special  quad- 
rangle, however,  the  rocks  of  the  upper  division  of  the  Potosi  directly  over- 
lie those  of  the  lower  division. 

T^e  rocks  of  the  lower  division  as  exposed  near  Creede,  are  all  highyl 
siliceous;  the  greater  part  are  Rhyolites,  although  quartz  latites  are  present 


in  the  northern  part  of  the  quadrangle  and  attain  a  great  development  to  the 
north  of  the  -quadrangle  in  both  forks  of  Willow  Creek.  The  rhyolites  and 
quarts  latites  alternate  and  it  is  probable  that  they  originated  from  differ- 
ent vents.  The  group  has  been  subdivided  into  five  formations,  two  rhyolites 
and  three  quartz  latites,  and  most  of  these  consist  of  several  flows  or  of 
flows  and  associated  tuffs.   The  lowest  quartz  latite,  the  Outlet  Tunnel 
quartz  latite,  underlies  a  part,  at  least,  of  the  Willow  Creek  rhyolite. 
Its  upper  surface  is  very  irregular  and  it  may  represent  a  much  older  period 
of  eruption  than  the  overlying  flows.   Campbell  Mountain  rhyolite,  wherever 
exposed,  immediately  overlies  a  rather  irregular  surface  of  the  Willow  Creek 
rhyolite.  The  second  quartz  latite,  the  phoenix  park  quartz  latite,  in 
general  overlies  the  Campbell  Mountain  rhyolite  but  in  places  is  interbedded 
with  it.  The  Equity  quartz  latite,  where  recognized,  immediately  overlies  the 
Campbell  Mountain  rhyolite,  and  it  is  believed  to  be  more  closely  related 
to  the  Phoenix  Park  quartz  latite  than  to  the  rhyolites. 

The  Outlet  Tunnel  quartz  latite. 

General  character  and  occurrence. 

The  Outlet  Tunnel  quartz  latite,  which  is  the  oldest  rock  exposed  in 
the  area  included  on  the  Creede  and  Vicinity  map,  is  a  chaotic  aggregate  of 
lava  flows  and  breccia  beds.   It  was  found  in  only  two  small  bodies,  in  the 
canyon  of  East  Willow  Creek  an  short  distance  north  of  the  Ridge  Mine,  and 
has  not  been  recognized  in  the  reconnaissance  of  the  adjoining  region.   In 
the  lower  body  it  gives  fair  outcrops,  although  the  contacts  are  everywhere 
covered  so  that  its  extent  and.  form  can  be  only  roughly  determined.  The 


upper  body  is  exposed  only  in  the  Outlet  Tunnel,  from  which  this  rock  receives 
its  name.   Talus  from  the  overlying  rhyolite  covers  the  lower  slopes  of  the 
hills  and  the  two  areas  may  be  connected  on  the  west  side  of  the  creek  beneath 
the  talus,  but  it  appears  more  probable  that  the  Willow  Creek  rhyolite  separates 
the  two  bodies  and  this  interpretation  is  shown  on  the  map.  Only  about  250  to 
300  feet  of  this  quartz  latite  are  exposed,  but  as  the  base  has  not  been  reached 
it  is  not  importable  that  the  part  exposed  represents  only  the  top  of  a  body 
of  considerable  thickness.  It  is  overlain  irregularly  by  the  Willow  Creek 
rhyolite,  although  the  contact  has  nowhere  been  seen. 

Petrography. 
Megascopic.-  The  rocks  both  of  the  flows  and  breccia  fragments  are  chiefly 

y 

biotite-hornblende-quartz  latites  of  purple-drab  to  gray  color.    They  carry 

I/  Since  the  color  is  rather  characteristic  of  many  of  the  rocks  of  the  district 
and  slight  differences  in  color  are  among  the  most  easily  recognized  differences 
between  some  of  the  rocks,  accurate  color  descriptions  of  the  rocks  are  highly 
desirable.  Accurate  color  names  require  a  standard  of  color  nomenclature  and 
no  such  standard  that  is  both  sufficiently  comprehensive  and  generally  accepted 
exists.  Robert  Ridgway's  "Color  Standards  and  Nomenclature",  published  in  1912, 
is  the  best  standard  that  we  have,  but  to  JBK  use  it  properly  requires  an  actual 
comparison  with  the  plates  in  Mr.  Ridgwayfs  book.  It  therefore  seemed  best  to  the 
authors  to  use  in  the  text  color  names  that  could  be  properly  understood  by 
reference  to  a  ^ood  dictionary  and  to  include  the  more  precise  name  according  to 
Mr.  Ridgway's  color  standards  in  foot  notes. 

According  to  Ridgway's  color  standards  the  rocks  of  the  Outlet  Tunnel  quartz 
latite  are  commonly  light  purple-drab  (lf"fb)  to  purple-drab  (!""  -),  less 
commonly  purplish  vinaceous  (!"•  b),  pallid  quaker  drab  (!•""  f),  or  pale 

neutral  gray  (-  -  d). 

. — » — .. ' 

phenocrysts  from  1  to  2  millimeters  across  of  white  plagioclase,  commonly  altered 
glassy  orthoclase,  quartz,  biotite,  and  altered  hornblende  in  an  aphanitic  ground 
mass;  a  few  show  in  addition  an  occasional  crystal  of  orthoclase  or  microperthite 
up  to  3  centimeters  across.  The  groundmass  equals  or  exceeds  the  phenocrysts  in 


amount.  The  rocks  are  mostly  rather  dense,  but  some  carry  visible  pores  and  gas 
cavities.  Fluidal  texture  is  often  presnt,  but  is  rarely  conspicuous.  Small 
inclusions  are  abundant  in  some  of  the  rocks  and  are  chiefly  of  quartz  latite 
of  darker  color  than  the  host  and  carrying  fewer  phenocrysts;  a  few  are  of 
rhyolites  and  andesites. 

Microscopic*-  The  microscopic  study  of  the  thin  sections  showed  that  the 
following  phenocrysts,  stated  in  the  order  of  their  abundance  as  roughly  es- 
timated -  plagioclase,  orthoclase,  quartz,  biotite,  green  hornblende,  magnetite, 
apatite,  titanite,  and  zircon  -  make  up  about  a  third  of  most  of  these  rocks. 
The  plagioclases  are  much  altered  to  calcite  and  saricite,  in  some  specimens 
to  kaolinite,  while  the  orthoclase  is  fresh;  the  quartz  Is  greatly  embayed 
from  magmatic  resorption;  the  apatite  crystals  have  colorless  ends  and  a 
faintly  pleochroic,  smoky,  yellow-brown  core.  The  phenocrysts  are  embedded 
in  a  matrix  which  is  indistinctly  polarizing  in  specks  and  shreds,  less  com- 
monly in  minute,  rounded  areas.  It  is  clouded  and  carries  numerous  minute 
reddish  shreds  which  are  probably  hematite. 

Less  common  rock  types.-  Rare  fragments  in  the  breccia  show  more  con- 
spicuous and  abundant  crystals  from  3  to  5  millimeters  across  of  white  plagio- 
clase, which  have  the  composition  of  andesine  and  of  biotite.   The  groundmass 
is  uneven  in  Size  of  grain  and  much  coarser  than  that  in  the  other  rocks;  it 
is  granophyric  or  micrographic  in  texture. 

Within  the  quartz  latite  breccia  are  thin,  irregular  bodies  of  rhyolite 

y 

flow  breccia.  This  rock  is  pale  purplish  gray   and  shows  a  few  crystals  of 
I/  Ridgway's  pallid  purplish  gray  (67  •••••  f). 

^«»  W<M  ^  ^M»M>  <»  mm  rnmmmm*^  mm^mm  mm  *mmm^  •»  —  ""  ^  *»«•  mmmmtmmmmmm  mm  mm  mm  mm**^  mm  mm  mm  +m>mm»mm,*mi  mmvmtmm  mmmmmmmmm 

quartz,  biotite,  and  glassy  crthoclase  about  a  millimeter  long  in  a  glassy  to 


apfcanitic  groundraass.  it  is  characteristically  porous  and  carries  very  abun- 
dant and  prominent,  irregular  shaped  inclusions  of  fibrous  pumice  up  to  several 
centimeters  across.  On  weathering  the  pumice  fragments  are  removed,  leaving 
large  ragged  cavities.  This  rock  closely  resembles  the  Tindy  Gulch  rhyolite 
breccia. 

Weathering  and  outcrops. 

The  rocks  are  considerably  altered  through  hydrothermal  action.  The 
plagioclase  is  sericitized,  and  the  hornblende  is  altered  to  a  crumbly  dark 
red  material;  secondary  calcite  and  chlorite  are  abunds^,  and  some  sulphides 
are  present.  This  rock  is  less  resistant  to  erosion  than  the  overlying 
rhyolite,  and  gives  inconspicuous  outcrops  poorly  exposed  through  the  talus 
from  the  cliffs  of  the  overlying  rhyolite. 

Willow  Creek  rhyolite. 
General  character  and  distribution. 

The  thick  series  of  flows  of  fluidal,  felsitic  rhyolites  characteristically 
exposed  above  Oreede  in  the  canyons  of  both  forks  of  Willow  Creek  is  here  called 
the  Willow  Creek  rhyolite.  It  is  also  prominent  in  Dry  Gulch,  in  Miners  Creek, 
and  above  the  Equity  mine  in  upper  West  Willow  Creek.  Its  distribution  is  shown 
on  Plate  II.  From  reconnaissance  work  about  Creede  it  is  believed  to  come  out 
from  under  the  overlying  rocks  to  the  north  at  Bondholder,  to  the  east  it  out- 
crops more  or  less  continuously  for  some  miles,  but  has  not  been  recognized  east 
of  Wagon  Wheel  Gap;  to  the  west  it  wedges  out  rapidly,  and  it  is  not  present 
to  the  south  of  Bristol  Head. 

On  the  west  slopes  of  Mammoth  Mountain,  nearly  2000  feet  of  this  formation 
are  exposed  and  the  base  is  not  seen;  in  other  parts  of  both  forks  of  Willow 


Creek  there  is  nearly  as  great  a  thickness.   Where  its  base  is  exposed  in  East 
Willow  Creek  above  the  Ridge  mine  it  overlies  an  irregular  surface  of  the  Outlet 
Tunnel  quartz  latite  and  is  not  over  200  feet  in  thickness,  it  therefore  de- 
creases in  thickness  from  nearly  2000  feet  to  about  200  feet  in  a  distance  of  a 
mile  and  a  half.  It  is  possible  that  the  Outlet  Tunnel  quartz  latite  represents 
a  lens  between  flows  of  the  billow  Creek  rhyolite,  but  this  is  not  believed 
probable. 

Petrography. 
Megascopic.-  The  rocks  of  these  flows  are  commonly  light  to  dark  purple 

i/ 

drab,  less  commonly  light  drab,  buff,  or  gray.    Fluidal  structure  is  always 

I/  Ridgway's  purple-drab,  ranging  from  pallid  to  dark  (1  fttf  f  to  1  tf ft  i) ; 
less  commonly  purple  vinaceous  (ltff  b),  light  cinnamon-drab  (13ftttb),  a 
number  of  pallid  shades  of  buff,  lilac,  drab,  and  gray,  and  the  lighter  shades 
of  gull-gray. 

present  and  is  commonly  prominent  and  characteristic.  The  main  part  of  the 
rock  is  dense  with  about  the  luster  of  freshly  broken  porcelain;  streaks  or 
lenses  are  decidedly  porous  and  have  a  paler  color.  These  streaks,  as  seen  on 
plates  broken  along  the  banding,  are  up  to  a  few  centimeters  wide  and  several 
decimeters  long;  they  are  seldom  over  a  few  millimeters  thick.  They  may  form 
as  much  as  10  per  cent  of  the  rock.   In  addition  to  these  larger  streaks  and 
grading  into  them  are  closely  spaced,  narrow  bands,  giving  the  rock  a  beautiful 
fluidal  structure.  In  places,  notably  in  Rat  and  Miners  creeks,  the  rock  has 
a  more  delicate  fluidal  structure  and  a  somewhat  paler  shade  of  color.  Near 
Weaver  the  rock  breaks  into  thin  plates  along  its  well  developed  and  closely 
spaced  fluidal  banding;  at  other  places  it  has  an  inconspicuous  fluidal  structure. 


In  a  few  places  it  carries  large  gas  cavities  lined  with  drusy  quartz  crystals 
and  partly  filled  with  chlorite,  rarely  it  carries  inclusions  of  foreign  rock. 

The  rock  carries  a  very  few  visible  crystals,  mostly  of  orthoclase,  but  with 
less  white  plagioclase,  commonly  kaolinized,  and  a  very  little  biotite.  Other- 
wise it  is  felsitic  or  aphanitic,  that  is,  its  composition  can  not  be  deter- 
mined in  the  hand  specimen,   There  are  believed  to  be  a  number  of  flows  present, 
but  their  close  similarity  and  the  lack  of  glassy  or  Other  recognizable  horizons 
at  the  top  of  base  of  flows  makes  it  difficult  to  distinguish  between  them. 
Where  the  greatest  thickness  is  exposed  in  Willow  Creek,  if  more  than  one  flow 
is  present  they  are  almost  identical  in  character  and  could  not  be  distinguished. 
Near  Sunnyside  several  flows,  some  of  them  different  from  the  rock  of  Willow 
Creek,have  been  inclusde  in  this  formation. 

Microscopic.-  The  rock  is  uniformly  holocrystalline  and  contains  few 
phenocrysts  chiefly  of  orthoclase,  with  some  plagioclase,  a  very  little  biotite, 
and  accessory  apatite,  iron  ore,  and  zircon.  The  plagioclases  are  albite  and 
albite-oligoclase,  and  are  commonly  kaolinized.   The  orthoclase  crystals  are 
commonly  broken  and  in  some  specimens  show  on  their  border  between  crossed 
nicols  a  lacework  grading  into  the  groundmass,  due  to  a  growth  of  material 
from  the  groundmass  on  the  crystals.   The  groundmass  is  beautifully  banded. 
The  main  part  is  indistinctly  polarizing  even  with  the  highest  magnification, 
and  is  clouded  from  minute  bodies  of  ferritic  material;  numerous  lenses  or 
streaks  are  much  coarser  and  are  clear.  These  coarser  streaks  correspond  to 
the  lighter  colored  bands  and  lenses  seen  in  the  hand  specimens  and  make  up 
a  small  per  cent  of  the  rock;  they  vary  in  width  from  a  few  millimeters  to  a 
small  fraction  of  a  millimeter. 


The  larger  lenses  show  a  marked  banding  and  concentration  of  quartz  in 

£ 

their  interior,  These  bans,  of  which  there  are  commonly  three  or  four  in  a 

lens  are  not  sharply  bounded  but  grade  into  each  other.  The  outer  band, 
which  is  not  alwqys  present,  is  a  few  tenths  of  a  millimeter  across,  and  is 
made  of  parallel  fibers  which  project  from  the  walls  and  extend  across  the 
band  resembling  delicate  spherulites.   Sharply  bounded  from  or  grading  into 
this  fibrous  band  or  forming  the  outer  band  of  some  of  the  lenses  is  a  band 
made  up  of  a  comparatively  coarse  crystallization  of  quartz  and  orthoclase. 
The  orthoclase  is  chiefly  in  fairly  well  formed  elongated  crystals,  occasion- 
ally greatly  elongated  or  acicular.  In  part  they  appear  to  have  grown  from 
the  walls  and  commonly  porject  out  from  the  walls;  in  part  they  are  embedded 
in  the  quartz  crystals  poikilitically.   They  vary  greatly  in  size,  and  the 
largest  are  as  much  as  0.3  millimeters  long.  The  quartz  is  interstitial  and  is 
occasionally  crystallographically  continuous  with  the  quart a  grains  of  the 
interior.   As  quartz  becomes  more  abundant  toward  the  interior  and  both  it 
and  the  orthoclase  become  more  coarsely  crystalline,  this  band  grades  into  the 
next  which  is  made  up  of  an  aggregate  of  interlocking  quartz  grains,  commonly 

a  millimeter  across,  with  some  gas  cavities*  Some  bands  have  a  core  which 

r 

is  largely  pore  space  into  which  project  drusy  crystals  of  quartz  and  ojctho- 

clase  which  can  be  easily  seen  with  a  pocket  lens.       Tridymite  was  not   ob- 
served.    Plates  \V  to "$£.  are  photomicrographs  of  thijj  sections  of  the  Willow 
Creek  rhyolite  showing  these  coarsely  crystalline  lenses,     plat e  U£  shows 
two  lenses  with  the  normal  groundznass  between;  Plate  V40r  shows  in  detail  one 
of  the  lenses  and  the  normal  groundmass  on  both  sides;  Plate  VI^F-  shows 
between  crossed  nicols  the  same  area  as  plate       ^<  P^ate  IS  sliet/a  anpa^t  °* 


-Mae  of  the  lu'oaflsg  lemierB^  Some  of  the  very  broad  lenses  do  not$rho¥/an  interior 
quartz  band,  but  its  place  is  taken  by  irregular  areas  of  quartz  grains  which  are 
scattered  through,  these  lenses*  -«b^«}iaszfc«*iHP4»fra~-«Btr  The  orthoclase  is  in 
general  irregularly  distributed.   The  smaller  lenses  do  not  show  so  distinct 
a  banding,  and  some  of  the  narrower  streaks  are  mere  strings  of  crystals.  In 
some  specimens  these  coarsely  crystalline  lenses  are  spherulitic  in  crystalli- 
zation or  micrographic,  with  little  if  any  concentration  of  quartz  in  the 
central  part.  These  lenses  are  less  clouded  than  the  main  groundmass  as  the 
ferritic  material  is  collected  in  small  grains  and  shreds  which  are  in  part 
black  and  opaque,  in  part  reddish  brown  and  translucent.  In  addition  there 
are  a  few  minute,  colorless  grains  with  a  rather  high  index  of  refraction 
and  without  perceptible  birefringence.   The  orthoclase  is  somewhat  clouded 
while  the  quartz  is  clear  except  for  scattered  streaks  and  irregular  areas  which 
carry  abundant  minute  gas  or  liquid  inclusions 4  no  liquid  inclusions  with  gas 
bubbles  were  found.   Phenocrysta  of  orthoclase  and  plagioclase  are  present 
in  some  of  the  larger  lenses. 

These  lenses,  which  differ  from  the  main  body  of  the  flow  in  their  more, 
porous  character,  coarser  crystallization,  euhedral  form  of  the  orthoclase  and 
banded  structure  evidently  represent  raagmatio  segregations  which  formed  before 
the  magma  came  to  rest.    Relief  of  pressure  due  to  the  extrusion  of  the  lava 
might  have  caused  the  mineralizers  to  concentrate  into  rounded  bodies,  perhaps 
in  part  as  bubbles  or  aggregates  of  bubbles  sealed  in  the  lava,  in  part  in 
solution  in  the  magma;  as  the  viscous  lava  flowed  these  bodies  were  drawn 
out  into  their  present  forms.  The  presence  of  the  mineralizers  hfcLd  back 
the  crystallization  of  these  streaks  until  after  the  main  body  had  solidified; 


; 


It  also  caused  the  coarser  crystallization,  the  concentration  of  the  quartz  in 
the  center  and  the  presence  of  the  gas  cavities.  On  account  of  the  coarseness 
of  the  crystallization,  the  euhedral  development  of  the  orthoclase,  and  its 
concentration  on  the  walls,  the  authors  believe  that  these  streaks  were  not 
highly  viscous  at  the  time  of  their  crystallization.  The  crystallization  of 
the  lava,  and  especially  of  the  coarser  lenses,  is  believed  to  have  depended 
more  on  the  loss  of  the  mineralizeSs  than  on  the  cooling  of  the  magma.  These 
mineralizers,  relaased  during  the  crystallization  of  the  magma,  may  have 
caused  the  kaolinization  of  the  plagioclase  which  is  almost  universal  and 
does  not  appear  to  be  due  to  weathering. 

These  streaks  show  considerable  resemblance  to  some  adularia  veins. 
The  concentration  of  orthoclase  on  the  borders,  and  the  interlocking  of  the 
quartz  grains  i  s  characteristic  of  both.  However,  the  form  of  the  orthoclase 
in  the  veins  is  rhomboidal,  while  in  the  streaks  of  the  rhyolite  it  is 
characteristically  prismatic.   They  also  show  a  resemblance  to  the  small, 

irregular  pegmatitic  bodies  which  are  common  in  some  granites. 

^HMJU 
Petrography.-  3we  chemical  analyses  of  this  rock,  madebby  W.  C.  Wheeler 

in  the  laboratory  of  the  United  States  Geological  Survey,  are  as  follows: 


Chemical  analyses  of  the  ~7illow  Creek  rhyollte. 


SiO 

I 
73.53 

II 

76.26 

III 
77.36 

A12°3 

12.87 

11.30 

11.37 

P62°3 

.88 

.52 

.31 

FeO 

.64 

.34 

.36 

MgO 

.56 

.02 

.14 

CaO 

.07 

.23 

.30 

Na2° 

.63 

2.81 

1.38 

K2° 

8.92 

6.77 

7.28 

H20  - 

.40 

.39 

.55 

H2° 

.70 

.14 

.26 

Ti02 

.19 

.15 

.16 

™2 

.23 

.19 

.06 

?2°5 

tr. 

.01 

.03 

S 

.02 

.26 

.33 

Cr  0 
2  3 

MnO 

none 
.09 

.05 

.03 

BaO 

.05 

.49 

.05 

99.87 

99.93 

99.97 

I.  Typical  fluidal  rhyolite  from  So  Ionian  Adit. 
II.  Typical  fluidal  rhyolite. 

III.  Typical  fluidal  rhyolite  from  Bachelor  Shaft,  Nelson  Adit,  about 
30  feet  from  vein. 


The  norms  of  these  analyses,  calculated  in  accordance  with  the  quant ita- 

U 

tive  system  of  Crossm  IddAngs,  Pirsson,  and  7/ashington,  are  as  follows: 

I/     Quantitative  classification  of  igneous  rocks,   Chicago  and  London,   1903. 


Norms  of  the  77illow  Creek  rhyolite. 

I  II                         III 

Quartz                    34.3  34.6                        41.5 

Corundum                £.0  1*0 

Orthoclase           52.8  40.0                       41.7 

Albite                     5.2  20.4                       11.5 

Anorthite               0.6  1.4 

Hyper sthene           1.8  .3 
Acmite 

Na.SiO 
2       3 

Wollastonite 
Pyrite 
Magnetite 
Ilmenite 

H2°*  C02'  etc* 


1.4 

0.4 

1.2 

0.5 

.6 

1.2 

0.3 

0.3 

.3 

1.4 

0.7 

1.2 

99.5         99.7          99.5 


Analysis  1  falls  in  Class  1,  order  3  near  order  4,  rang  1,  and  subrang  1, 
3(4).I.I),  and  is  bisbose.   Chemically  this  rock  is  remarkable  for  its 
high  silica  and  potash  with  low  soda  and  very  low  lime.   Analysis  2  falls  in 
Class  1,  order  4  near  3,  rang  1,  and  subrang  2  (1. (3)4.1.2(3)  and  is  an  omeose. 
It  differs  from  analysis  1  chiefly  in  the  greater  amount  of  soda;  it  is  re- 
markable that  the  BaO  in  this  rock  exceeds  the  CaO;  this  may  be  due  to  the 
presence  of  a  small  amount  of  secondary  barite.   Analysis  3  falls  in  the 
madgeburgose  (1.3.1.2)  and  is  higher  in  silica  than  the  other  two,  but  is 
otherwise  intermediate  between  them.  In  all  these  rocks  the  norm  differs 
from  the  modCchiefly  in  the  absence  of  corundum  and  the  presence  of  biotite 

instead  of  pyroxene. 

j^ 
The  -tsafr  analyses  differ  somewhat  more  than  would  be  expected  from 

specimens  collected  from  the  same  flow.   Analysis  i  shows  lower  Si02  and 
especially  lower  soda  and  higher  potash.  This  may  not  represent  an  original 
difference  in  the  rocks  but  may  indicate  that  the  rock  represented  by 
Analysis  1  has  had  its  plagioclase  altered  to  kaolinite.   The  considerable 
amount  of  water  shown  in  the  analyses  and  the  presence  of  corundum  in  the 
norm  bear  out  this  suggestion.  It  is  possible  also  that  adularia  has  been  added, 

Structure. 

The  strike  and  dip  of  the  fluidal  banding  of  the  rock  varies  rapidly 
from  place  to  place  an  an  irregular  manner;  the  dip  is  not  uncommonly  steep. 
So  far  as  could  be  observed  this  is  not  due  to  tilting  subsequent  to  the 
extravasation  of  the  lava  but  to  the  irregular  flow  of  a  viscous  magma 
probably  over  an  uneven  surface.   A  nearly  vertical  sheeting  duCto  crushing 


or,  perhaps,  in  part  to  shrinkage  on  cooling,  is  commonly  present  and  locally 
it  is  close  spaced  and  prominent.   Where  two  or  more  such  sheetings  are 
developed  the  rock  breaks  into  rude  columns  on  pencils.  Plate  glllo  aad  ^|flll 
shows  the  details  of  a  typical  outcrop;  they  picture  the  cliffs  about  the 
Monte  Carlo  mine  on  the  southeast  slope  of  Campbell  Mountain.  The  broken  cliffs 
above  the  talus  are  about  a  thousand  feet  high. 

Weathering  and  alteration. 

The  weathering  of  this  rock  has  been  largely  mechanical  and  due  to  frost 
action,  and  differential  expansion  caused  by  temperature  changes  which  are 
rather  extreme  in  this  high  altitude.   Gravity  has  also  been  an  important 
factor  and  rock  slides  of  various  sizes  are  very  common.   T^e  prominent 
fluidal  structure  and  the  sheeting  have  greatly  influenced  the  breaking 
down  of  the  rock.  Soil  is  scant  and  great  piles  of  small  rock  fragments 
made  up  largely  of  small  plates  of  nearly  fresh  rock  are  characteristic  at 
the  base  of  the  cliffs.  The  gentler  slopes  of  the  highlands  show  a  scant 
soil  with  abundant  fragments  of  nearly  fresh  rock  scattered  through  it. 

Near  the  mineral  veins  or  other  sulphide  bodies  the  rock  has  been  more 
or  less  altered  by  the  mineral  solutions.  In  places  near  the  surface  along 
the  Amethyst  vein  there  are  considerable  bodies  of  white,  kaolinized  rock, 
probably  due  to  leaching  by  acid  solutions,  in  addition  mush  of  the  rock 
while  showing  no  other  signs  of  alteration,  has  its  phenocrysts  of  plagio- 
clase  altered  to  kaolin.  This  kaolinization  shows  no  relation  to  the  zone 
of  weathering  nor  to  zones  of  mineralization  and  is  believed  to  have  taken 
place  immediately  after  the  solidification  of  the  lavas  and  before  complete 
cooling. 


Topography  and  soenery* 


The  rock  commonly  outcrops  in  jagged  and  more  or  less  broken  cliffs  with 
great  talus  heaps  at  their  base.   Of  all  the  rocks  in  the  area  it  is  exceeded 
in  hardness  and  resistance  to  weathering  only  by  the  latite  of  MacKenzie 
Mountain,  and  most  of  the  rugged  topography  and  deep  canyons  about  Creede  are 
in  this  rock*  Wherever  the  streams  have  cut  deeply  into  it  they  have  sharp  ax 

t 

canyons  or  gorges  with  steep  rugged  walls  and  brotaen  cliffs  for  a  thousand 

feet  or  more.   The  uniformity  of  the  material  does  not  lead  to  the  development 

3^         *  ^ 
of  benches*  Plates^,  JMfr  1.T,  ajftd  fflll  show  characteristic  outer-ops  of  this 

rock.  Plates  XI  and  Xlla  are  photographs  looking  up  the  creek  from  a  point  a 
few  hundred  yards  below  the  forks  of  Willow  Creek.  The  cliffs  are  all  of 
Willow  Greek  rhyolite,  that  in  the  center  between  the  forks  of  Willow  Creek 
is  about  1500  feet  high.  The  talus  slopes  at  the  base  of  the  cliffs  are  in 
all  cases  at  about  the  top  of  the  Willow  Creek  rfcyolite  and  the  timbered 
slopes  on  the  right,  above  the  shoulder,  show  the  characteristic  topograpny 
of  the  Campbell  Mountain  rhyolite.  Platelj  Klla  abd  }flll  show5  more  detailed 
views  of  these  cliffs.   They  were  taken  from  a  point  about  five  hundred  feet 
below  the  Mollie  S.  Mine  and  show  the  cliffs  on  the  west  side  of  East  Willow 
Creek.   These  cliffs  are  about  1500  feet  high.  The  Monte  Carlo  Mine  building 
is  indistinctly  shown  near  the  top  of  the  cliffs  to  the  right  of  the  center 

3SE 

of  plate  VlII.   PlatelFshows  the  canyon  of  Willow  Creek  above  Creede  which 

is  out  in  this  rock.   These  rugged  outcrops  are  especially  developed  in 

both  forks  of  Willow  Creek  and  in  Dry  £ulch.   In  Miners  Creek  they  are  nearly 

as  characteristic. 


\ 


3 


' 


Campbell  Mountain  rhyolite, 


General  character  and  distribution* 

Overlying  the  Willow  Creek  rhyolite  rather  irregularly  is  a  rhyolite 
flow  breccia  which  is  here  called  the  Campbell  Mountain  rhyolite.    In 
most  places  no  evidence  was  seen  of  more  than  one  flow  but  in  East  Willow 
two  flows  of  this  type  are  separated  by  a  few  hundred  feet  of  Phoenix 
Park  quartz  latite.   It  is  present  on  both  sides  of  Miners  Creek,  in  lower 
Rat  Creek  drainage,  and  northeast  of  Monon  Hill.   It  was  also  found  in 
Windy  Gulch  Just  south  of  Bachelor.   Several  isolated  outcrops,  partly 
bounded  by  faults,  are  present  on  both  sides  of  Willow  Creek,  a  short 
distance  above  Creede.   It  caps  parts  of  Mammoth  Mountain.   A  narrow 
band  extends  f r  m  the  ridge  north  of  Mammoth  Mountain  in  a  northwesterly 
direction  and  crosses  East  Willow  Creek  at  Phoenix  Park;  it  continues 
to  the  southwest  to  Campbell  Mountain.   Just  northwest  of  this  it  is 
cut  out  for  a  short  distance  by  the  Mammoth  Mountain  rhyolite  but  comes 
in  again  as  a  thicker  member  in  Nelson  Creek  and  to  the  northwest.  It 
has  been  recognized  on  both  sides  of  the  Equity  fault.   It  is  not  ex- 
posed to  the  west  of  test  Willow  Creek  except  near  its  mouth,  but  in  the 
underground  workings  along  the  Amethyst  vein  this  rhyolite  forms  the 
hanging  wall.   Ofce  details  of  the  distribution  in  the  Creede  Special  area 
are  shown  on  Plate  II.   0?o  the  west  of  the  Creede  and  Vicinity  area  it  is 
well  developed  in  the  drainage  of  Shallow  Creek  but  is  not  present  south 
of  Bristol  Head.  It  has  been  recognized  as  far  east  as  Wagon  tfheel  Gap 
and  as  far  north  as  Bondholder  in  Spring  Creek.   It  has  not  been  recognized 
to  the  south. 


About  a  quarter  of  a  mile  a  few  degrees  south  of  east  of  the  Captive 
Inca  shaft  in  a  great  landslide  area,  at  the  face  of  short  tunnel,  is  an  ex- 
posure, believed  to  be  in  place,  of  a  somewhat  altered  rhyolite  flow  breccia 
or  tuff  which  probably  belongs  to  this  rhyolite  although  it  has  more  of  the 
appearance  of  some  of  the  roc*  of  the  Windy  Suloh  rhyolite  breccia.  The  lat- 
ter rock  is  not  known  in  this  part  of  the  quadrangle  and  if  the  exposure  does 
not  belong  to  the  Campbell  Mountain  rhyolite  it  probably  belongs  to  the  over- 
lying tuff,  in  which  case  it  may  be  a  large  block  of  slide  rock. 

The  Campbell  Mountain  rhyolite  varies  greatly  in  thickness  and  for  three 
reasons:  it  flowed  over  a  rather  uneven  surface  of  the  underlying  rhyolite; 
in  most  places  erosion  had  cut  deeply  into  it  before  the  overlying  rocks  of 
the  upper  division  of  the  potosi  were  extruded;  it  thins  out  and  the  flows 
wedge  out  between  flows  of  the  Phoenix  Park  latite  in  the  northeast  corner 
of  the  quadrangle.  The  maximum  thickness  is  south  of  Nelson  Mountain  where 
nearly  a  thousand  feet  are  present  and  it  was  probably  thicker  since  the  top 
is  a  surface  of  erosion;  less  than  a  mile  to  the  south  it  is  entirely  eroded 
and  the  rocks  of  the  upper  division  rest  directly  on  the  Willow  Creek  rhyolite. 
To  the  west  of  the  quadrangle  in  Shallow  Creek  the  thickness  is  probably  even 
greater;  although  a  few  miles  farther  west,  south  of  Bristol  Head,  this  flow 
wedges  out  and  the  lower  division  is  made  up  entirely  of  quartz  latite. 

Contacts* 

The  basal  contact  of  the  Campbell  Mountain  rhyolite  with  the  Willow 
Creek  rhyolite  where  observed,  is  commonly  sharp  and  shows  evidence  of 
some  erosion  preceding  its  extrusion.  West  of  Willow  Creek,  above  Creede, 
at  the  Equity  Mine  the  contact  is  well  exposed,  especially  in  the  mine  -¥ 
workings.   It  dips  steeply  to  the  west.   The  two  rhyolites,  each  in  typi- 
cal development  show  a  sharp  contact  and  are  closely  adherent.  The  over- 
lying Campbell  Mountain  rhyolite  has  plucked  off  numerous  blocks  from  the 


underlying  Willow  Creek  rhyolite  and  carries  the  latter  as  angular  inclu- 
sions up  to  a  foot  across.   The  Campbell  Mountain  rhyolite  is  not  noticeably 
different  at  the  contact  from  the  normal  rock  except  for  the  presence  of 
the  numerous,  rather  large  included  blocks  of  the  underlying  Willow  Creek 
rhyolite  and  these  extend  for  only  a  few  feet  from  the  contact.  At  other 
places  the  two  rocks  appear  to  grade  into  each  other  and  even  with  con- 
tinuous exposures  there  is  up  to  a  hundred  feet  of  rock  of  intermediate 
character  between  the  two  typical  rocks,  as  it  well  shows  on  the  East  side 
of  East  Willow  Creek  about  half  a  mile  above  the  Ridge  Mine.  In  Miners 
and  Rat  creeks  also  some  difficulty  was  experienced  in  locating  the  con- 
tact, partly  on  account  of  the  lack  of  a  sharp  contact  and  distinctive 
character  of  the^Sgo  members,  and  partly  on  account  of  poor  exposures. 
The  upper  contact  is  everywhere  sharp  but  some  of  the  overlying  rocks  so 
closely  resemble  the  Campbell*  Mountain  rhyolite  that  the  separation  was 
somewhat  difficult. 

petrography. 
Megascopic*-  In  color  the  Campbell  Mountain  rhyolite  is  commonly  red- 

y 

dish  brown  or  drab  ,  has  a  rather  dull  luster,  a  delicate,  indistinct 

I/  Ridgway»s  livid  brown  (!•»••-),  purple-drab  (!»»•'-},  light  purple 
drab  (lfl»«b)f  cinnamon  drab  (13"»f-)  or  light  quake r  drab  (l"fftb). 

fluidal  texture  and  a  characteristic  mottled  or  spotted  appearance  due  to 
inclusions  of  lighter  and  darker  shade.  It  is  generally  porous  with  pores 
less  than  a  millimeter  across;  the  larger  ones  are  lined  with  drusy  crystals 
of  quartz  and  feldspar  or  are  partly  filled  with  a  network  of  minute  crystals 


arranged  in  strings  like  rock  candy.   The  inclusions  are  in  part  angular 
fragments  of  a  somewhat  darker  colored  quartz  latite  similar  to  the  Outlet 
Tunnel  quartz  latite;  they  are  rarely  over  a  centimeter  across  and  are 
nearly  always  much  altered.   Fragments  of  the  underlying  Willow  Creek 

rhyolite  are  abundant  only  near  the  contact  with  that  formation;  occasional 

*/ 

fragments  of  andesites  and  other  ^ocks  are  present.   Ever  more  abundant  and 

characteristic  are  the  rounded  to  subangular  fragments  of  rhyolite  which 
differ  from  the  host  chiefly  in  their  more  porous  character,  slightly 
paler  shade  of  color,  and  coarser  crystallisation.  So^e  of  them  have 
irregular  serrated  outlines,  some  are  flattened  parallel  to  the  flow 
structure  or  rudely  lenticular.  They  are  commonly  very  ragged  and  as  seen 
in  the  specimens  fray  out  at  their  ends.  They  are  believed  to  represent, 
in  part  at  least,  magmatic  segregations  not  very  different  from  the  lighter 
lenses  in  the  Willow  Creek  rhyolite,  but  they  may  represent  fragments  of  the 
magma  which  had  already  crystallized  and  were  torn  from  the  sides  of  the 
vent  by  the  viscous  lava.  Both  kinds  of  inclusions  are  commonly  bordered 
by  a  narrow  band  of  somewhat  paler  shade  than  the  main  rhyolite  mass. 

A  rock  of  somewhat  different  appearance  is  prominent  in  the  body  south 

of  Nelson  Mountain  and  is  also  present  near  Sunnyside  and  in  other  places. 

I/ 
This  is  light  gray  to  white  and  has  a  delicate  flow  structure.  It  carries 

I/  Ridgway's  gull  gray  (d(8))  light  gull  gray  (f(9),  pale  mouse  gray 
(15»t»»»  <j.)f  or  white. 

.»_•        •.•«-•_•»« ._».•»••••..•«.•.•  —  »•  —  •»..- .•.•••.»—.»»»•  ••••«••••••••»  —  ••  —•••  —  —•  —  ••••••«•—•••-'"••-•••"  «»«•••••  •—•..»  —  —  »--••—•» 

numerous  included,  flat,  splinter- like  fragments  of  a  white,  rather  porous 
rhyolite,  in  addition  to  some  of  quartz  latite  and  Willow  Creek  rhyolite; 


they  tend  to  have  their  maximum  diameter  parallel  to  the  flow  structure. 
It  differs  from  the  drab  type  chiefly  in  its  color,  but  also  has  a  more 
prominent  fluidal  structure  and  somewhat  more  conspicuous  and  abundant 
included  fragments  which  are  in  this  type  commonly  porous.  It  is  not 
believed  to  be  a  distinct  flow  but  rather  the  top  or  other  part  of  one 
or  more  flows  of  the  normal  rock.  It  is  everywhere  closely  associated 
with  the  normal  rock  and  south  of  Nelson  Mountain  it  makes  up  most  of  the 
upper  part  of  the  body  although  it  is  not  confined  to  any  particular 
horizon.  In  places  rocks  transitional  between  the  two  were  found.  Locally 
leaching  of  the  typical  rock  has  given  rise  to  a  rook  similar  to  the  gray 
type  and  it  may  have  developed  much  of  this  type. 

Both  types  show  a  few  one  millimeter  sized  crystals  of  glassy  ortho- 
clase  and  white  plagioclase  and  rare  crystals  of  biotite  and  quartz  in  an 
aphanitic  groundmass.   Glassy  or  highly  vesicular  layers  have  not  been 
recognized. 

Microscopic."  The  thin  sections  show  that  the  rocks  differ  but  little 
from  those  of  the  underlying  pillow  Creek  rhyolite;  on  the  whole  they  have 

somewhat  more  abundant  and  more  calcic  plagioclase,  some  (fuarts  phenocrysts 

i  F 


and  a  slightly  different  grAoundpass  but  individual  sections  can  not  be 

*    / 
easily  distinguished.   They  carry  scattered  phenocrysts,  chiefly  of  or- 

thoclase  with  some  calcic  andesine,  embayed  quartz,  and  biotite;  and 
accessory  apatite,  iron  ore,  and  zircon.  The  groundmass  is  delicately 
fluidal,  most  of  it  exceeding  finely  crystalline  to  submicroscopio  with 
streaks  and  irregular  patches  of  coarser  spherulitio  or  granophyric 
crystallization.   The  blotches  so  prominent  in  the  hand  specimens  are 
sharply  bounded  from  the  host  and  are  somewhat  coarser  in  crystallization 

but  otherwise  similar. 


Weathering  and  outcrops. 

This  rock  breaks  up  readily  under  the  influence  of  weathering  into 
fragments,  flattened  parallel  to  the  flow  banding  and  commonly  several 
inches  across;  it  gives  poor  outcrops  and  commonly  forms  gentle  slopes 
strewn  with  fragments  of  the  underlying  rock.  The  rock  is  much  less 
resistant  than  the  underlying  Willow  Creek  rhyolite  and  to  this  differ- 
ence is  due  much  of  the  topography  and  scenery  on  both  sides  of  Willow 
Creek  above  Creede.   !Phe  cliffs  are  almost  entirely  of  the  Willow  Creek 
rhyolite  and  the  soft,  gentle  slopes  above  most  of  the  benches,  and  the 
flat  tops  of  the  ridges  are  commonly  formed  from  the  Campbell  Mountain 
rhyolite.  The  marked  difference  between  the  outcrops  of  the  two  formations 
adds  greatly  in  distinguishing  between  them,  and  in  most  places  the  contact 
can  be  approximately  mapped  from  the  change  in  topography.   In  plateJSf 
opposite  p.    ,  on  the  left  of  the  canyon  the  gentle  talus  covered 
slopes  above  the  cliffs  of  Willow  Creek  rhyolite  are  of  Campbell  Mountain 
rhyo  li  te^^wa^ar«-^tie*^ite--s4mi^aa«-^10pas  "•abooFe^Wa*-  te£tt*-^*~^to^&:&)^ 
invPlAt*  In  PlatejH,  on  the  nose 


between  the  forks  of  Willow  Creek,  the  talus  -covered,  timbered  slope  between 
the  two  cliffs  is  formed  of^^f  Campbell  Mountain  rhyolite  overlying  the 
lower  cliffs  of  Willow  Creek  rhyolite  and  fagged  on  the  north  against  the 
upper  cliffs  of  Willow  Creek  rhyolite. 


i^eipplqg^g^ggp^;^^ 


Distinction  from  other  rhyolites. 

Locally  the  Campbell  Mountain  rhyolite  is  very  difficult  to  distinguish 
from  the  Willow  Greek  rhyolite;  it  also  resembles  some  parts  of  the  Windy 
Gulch  rhyolite  breccia  and  the  Mammoth  Mountain  rhyolite*  The  compariosn 
withv  the  Willow  Creole  rhyolite  will  be  taken  up  in  the  folio  /ing  paragraph: 
compariosn  with  the  Windy  Gulch  and  Mammoth  Mountain  rhyolites  will  be  left 
until  they  have  been  described 

Th©  normal  drab  flow  breccia  of  the  Campbell  Mountain  rhyolite  is 
easily  distinguished  from  any  part  of  the  Willow  Creek  rhyolite.  Moreover, 
the  normal  fluidal,  banded  Willow  Creek  rhyolite,  as  exposed  above  Creede, 
is  distinct  from  any  part  of  the  Campbell  Mountain  rhyolite,  yet  some  less 
typical  kinds  of  what  is  believed  to  be  the  gray  rock  of  the  Campbell 
Mountain  rhyolite  can  not  with  certainty  be  distinguished  from  some  of  the 
finely  fluidal  parts  of  the  Willow  Creek  rhyolite.   In  the  drainage  of 
both  forks  of  Willow  Creek,  except  near  Phoenix  Park,  the  two  rocks  are 
easily  distinguished  and  in  places  at  least  the  contacts  are  sharp. 
However,  in  some  places  near  Phoenix  Park  as  much  as  100  feet  of  rock 
of  doubtful  character  waa  found  between  the  typical  Campbell  Mountain 
rhyolite  and  the  typical  Willow  Creek  rhyolite  and  there  appears  to  be 
a  dradation  between  them.  In  the  drainage  of  Rat  and  Miners  creeks 
there  is  considerably  more  rock  whose  posit ionls  uncertain  and  the  boundary 
between  the  two  rhyolites  of  this  area  could  not  be  followed  precisely. 
It  is  believed  that  a  flow  of  delicately  fluidal  rhyolite  carrying  a 
few  included  fragments  lies  between  the  normal  coarsely  fluidal  Willow 


Creek  rhyolite  and  the  typical  Campbell  Mountain  rhyolite.  This  has  been 
included  in  the  Willow  Creek  rhyolite  which  it  most  resembles.  However, 

poor  exposures,  alteration,  and  a  variability  in  the  two  rocks  which  in 

un- 
some  places  closely  resemble  each  other  have  made  the  mapping/satisfactory 

in  places.   In  general  the  Campbell  Mountain  rhyolite  is  a  flow  breccia 
with  abundant  included  fragments  and  a  faintly  developed  flow  structure 
while  the  Willow  Creek  rhyolite  carries  few  inclusions  and  has  a  prom- 
inant  and  commonly  coarse  fluidal  texture. 

Phoenix  park  quartz  latite. 
Sonera!  character  and  distribution. 

The  Phoenix  Park  quartz  latite  is  made  up  of  lava  flows  and  tuff 
breccias  of  a  fairly  uniform  rock;  it  receives  its  name  from  its  devel- 
opment about  Phoenix  Park.  It  is  well  developed  to  the  north  of  the 
Creede  special  quadrangle  where  it  is  made  up  largely  of  great  lava 
flows  but  within  the  mapped  area  it  is  chiefly  breccia  and  tuff  with 
massive  flow  rock  in  smaller,  less  continuous  bodies.  The  breccia  is 
chaoitc,  shows  little  bedding  or  sorting,  and  consists  mainly  of 
rounded  or  subangular  fragments,  many  of  them  several  feet  across. 
They  are  nearly  all  of  quartz  latite  similar  to  that  of  the  flows. 
Fine  tuffs  are  only  exceptionally  present. 

This  quartz  latite  was  found  only  in  the  upper  part  of  East  Willow 
Creek.  The  main  body  overlies  a  fairly  regular  surface  of  the  Campbell 
Mountain  rhyolite  and  is  overlain  irregularly  by  the  Mammoth  Mountain 
rhyolite.   Another  body  of  similar  rook  occurs  as  a  lens  between 


flows  of  the  Oaopbell  Mountain  rhyolite.  It  is  believed  that  here  there 
was  an  interbedding  of  the  two  types  of  material.   This  type  of  rock 
makes  up  the  greater  part  of  the  upper  division  for  many  miles  about 
Creede. 

This  rock  wedges  out  to  the  south  and  thickens  rapidly  to  the  north. 
The  maximum  thickness  within  the  quadrangle  is  about  five  hundred  feet, 
but  a  few  miles  to  the  north  it  is  very  much  thicker. 

Petrography. 
Megascopic.-  In  color  the  rocks  are  commonly  red-brown  to  pale 

y 

reddish  drab.   They  are  quartz  latites  and  carry  rather  abundant  vis- 
ible crystals,  about  a  millimeter  in  cross  section,  of  white,  striated 

***•-•*»»••••••— ^  .^ -»•  ^  •  *»—.-»-»**  <^  —  **>  •••  -M  .»••«  .MlBW  ^  .*»  W  *••  M»  «••<•»*»  -w  ••  «w  Ml  ••-«»«»  "»»  ^  •••«•]•«•  M»  «»«»*••«•»  ;M»  *»•»•»  M  M  ^  *V  «»  M  «W  <*-  «»  M»  <M 

I/  Ridgwayfs  purplish  or  brownish  vinaceous  (lt>fb  to  3ftlb)  to  light 
purple  or  light  vinaceous  drab  (llfttb  to  S'^'b),  deepening  to  livid 
brown  (llff-)  or  dark  purple -drab  (ltftli)  or  becoming  as  light  as 
pallid  vinaoeous  drab  (5tfftg). 

plagioclase,  and  glassy  orthoclase  in  about  equal  amounts,  less  of 
glassy  quartz,  hexagonal  plates  of  black  biotite  and  prisms  of  black 
hornblende  and  a  few  of  pale  yellow,  lustrous  titanite.   These  crystals 
are  imbedded  in  a  rather  dense  to  highly  porous  aphanitic  groundmass. 

a  few  feet  at  the  base  of  some  flows  the  phenocrysts  are  imbedded 
in  a  black  glass.   Many  of  the  rocks  carry  inclusions  of  latites  of 
somewhat  different  color  and  texture,  and  some  of  rhyolite,  andesite, 
quartz it e,  and  granular  rocks* 

In  addition  to  the  quartz  latites  there  are  a  few  thin  irregular 


flows  of  a  pale  purplish  vinaceous  rhyolite  flow  breccia  which  are 
I/  Ridgway's  (!•••  b). 

characterized  by  abundant  inclusions  of  fibrous  pumice*   A  few  crystals 
of  glassy  orthoclase  and  biotite  are  present  in  a  rather  porous  aphan- 
itic  groundmass.  This  rhyolite  is  very  similar  to  that  characteristic 
of  the  Windy  (Julch  rhyolite  breccia* 

Microscopic*-  The  microscopic  study  shows  that  the  quartz  latites 
are  porphyritic,  and  that  phenocrysts  make  up  nearly  half  of  the  rook. 
T^ese  phenocrysts,  which  vary  in  cross  section  from  2  millimeters  down 
to  a  fraction  of  a  millimeter  and  are  somewhat  broken,  include  plagioclase, 
quartz,  orthoclase,  biotite,  hornblende,  and  titanite  with  accessory  iron 
ore,  apatite,  and  zircon.  The  plagioclase  is  in  well  formed,  nearly 
equant  crystals;  they  are  zoned  and  vary  from  andesine  to  labradorite, 
averaging  andesine-labradorite.  The  hornblende  is  a  deep  brown  variety 
in  some  sections,  light  green  in  others,  and  rather  dark  olive  green  in 
still  others,  although  commonly  nearly  as  abundant  as  biotite  it  was  not 
found  in  some  of  the  sections*  Titanite  is  in  crystals  a  millimeter 
across  and  is  nearly  as  abundant  as  the  hornblende.  The  groundmass  is 
very  find  textured  and  in  part  spherulitic,  in  part  indistinctly  polar- 
izing and  is  rhyolitic  in  character.  It  is  clouded  and  in  addition  is 
dusted  with  minute  incousions,  probably  of  hematite.  The  rocks  are  com- 
monly fresh  but  some  are  altered,  probably  by  hydrothermal  agents  and 
show  secondary  calcite,  sericite,  chlorite,  and  sulphides. 


Outcrops  and  weather ing. 

The  breccia  beds  are  rather  easily  broken  down  by  weathering  and  on 
the  whole  the  Phoenix  Park  quartz  latite  is  somewhat  less  resistant  than 
the  overlying  and  underlying  formations.  Llany  of  the  outcrops  are  rather 
poor  and  are  more  or  less  covered  with  talus  or  glacial  material.  Horth 
of  Phoenix  Park  flows  make  up  most  of  the  rock  and  the  outcrops  are  better. 
T  the  north  of  the  quadrangle  where  great  flows  make  up  most  of  this 
formation  the  outcrops  are  much  better  and  the  very  rugged  mountain  and 
canyons  about  San  Luis  Peak  have  bean  carved  out  of  these  flows*  Weather- 
ing is  due  chiefly  to  mechanical  agencies  and  in  many  places  the  rock 
breaks  up  into  a  crumbly,  sandy,  mass,  although  the  minerals  are  still 
comparatively  fresh.  Much  of  the  area  made  u  p  of  this,  rock  has  been 
glaciated. 

Equity  quartz  latite. 
General  character  and  occurrence. 

ThQ  Equity  quartz  latite,  which  is  made  up  entirely  of  massive  rock 
and  in  large  part,  at  least,  of  a  single  great  flow,  is  named  from  its 
prominent  development  near  the  Equity  mine.  It  overlies  rath^a)  regularly 
the  Campbell  Mountain  rhyolite  and  therefore  occupies  aboutk  the  same 
position  in  the  section  as  the  Phoenix  Park  latite  from  which  it  differs 
chiefly  in  its  more  massive  character  but  also  slightly  in  its  composition 
whjc^is  somewhat  nearer  that  of  a  rhyolite.  The  two  latites  have  not 
been  found  in  contact  but  they  are  believed  to  be  very  closely  related 
and  to  represent  different  phases  of  the  same  period  of  eruptive  activity. 


This  rock  was  found  in  upper  West  Willow  Creek  from  TTpper  Deerhorn 
Creek  to  and  above  the  Equity  Mine.  It  caps  the  high  ridge  north  of  the 
Equity  fault.  Just  south  of  the  Equity  fault  it  has  a  thickness  of  about 
a  thousand  feet  with  an  erosion  surface  at  the  top. 


slide,  or  |other  debris,  and  their  mapping  is  not  accurate.  On  the  crest 
of  the  ridge  east  of  the  Equity  Mine  and  north  of  the  fault  its  base  is 
well  exposed;  here  it  overlies  the  Campbell  Mountain  rhyolite  rather 
irregularly.  Just  south  of  the  Equity  fault  the  base  is  mapped  only 
approximately  as  talus  and  slide  completely  cover  this  area.  The 
lowest  exposures,  which  are  about  200  feet  above  the  bed  of  the  creek, 
are  of  the  Equity  quartz  latite;  the  first  exposure  in  the  Equity  tunnel 
is  of  the  Campbell  Mountain  rhyolite;  otherwise  the  mapping  of  this 
contact  is  based  on  the  topography.  On  the  west  side  of  the  creek, 
only  a  few  feet  above  the  creek  bed,  are  good  exposures  of  this  latite. 
North  of  the  Equity  Mine  the  trench  of  the  creek  is  in  latite  while  the 
slopes  to  the  east  of  the  flat  are  in  rhyolite,  believed  to  be  the 
•/illow  Creek  rhyfllite;  there  must  be  a  fault  between  the  two.    The 
western  boundary  of  the  mapped  body  is  also  believed  to  be  a  fault 
although  it  is  in  an  area  lacking  in  exposures. 


Petrography. 

i/ 

Megascopic,-  The  fresh  rock  is  commonly  light  Quaker  drab.    The 

"^^^"^^**^^*  ^  •"»"•  ^  *•»«•»«*  «W  MM  »—  •»  «•  MM  ^  M»  *»«-•*  >VM»^  •*«•.*»  MB  MM  *•  «M  WW  «•  *»  ^  Mb  "—  M  ^  «•  W* 

I/  Ridgway*s  light  quaker  drab  (1«»««  b)  to  pale  purplish  gray  (67»»'»»d), 
less  connonly  light  olive  gray  (25ttf»fd),  light  mouse  gray  (15»'»»»b), 
dawn  gray  (15fftrd),  or  lighter  or  darker  shades  of  any  of  these  colors. 

"•"•——^— ••••••-••-•"•••••———-••—••— ————^— —————— —-.——•— —.•———.«.— .—•——»— •—•.••^«— •»„«•_„__„» 

hand  specimen,  shows  rather  abundant  crystals  of  white  plagioclase,  and 
some  of  quartz,  glassy  orthoclase,  and  biotite  in  a  groundmass  which  is 

fairly  dense  and  shows  inconspicuous,  delicate  fluidal  texture.  In 

2/ 

addition;a  few  of  the  rocks  show  bodies  of  a  pale  olive  gray  color  up 

2/  Ridgway's  pale  olive  gray  (23ftff). 

U  — -_—-  —  —._  —  -.  —  -  —  —  -  —  --._  —  —  —  -.  —  --.  —  —  —  -  —  —  _  —  _  —  —  _._«.—«.«.  —  _„-,  —  «_  —  —.____-. 

to  five  millimeters  wide  and  a  few  centimeters  long  with  a  rudely  len- 
ticular form  but  much  serrated  in  detail.   They  are  of  coarser  crystalli- 
zation but  are  not  perceptibly  porous.  The  rock  is  everywhere  somewhat 
altered;  the  plagioclase  and  dark  minerals  are  commonly  largely  replaced 
by  calcite,  chlorite,  and  sulphides.  The  base  of  the  flow  where  exposed 
shows  a  few  feet  of  dark  glass  in  which  are  imbedded  the  usual  crystals. 
On  the  upper  parts  of  the  ridge  included  on  the  map  in  the  Equity 

quartz  latite  is  a  rock  which  more  closely  resembles  the  Phoenix  park 

3/ 
quartz  latite.  In  color  it  is  somewhat  brighter  than  the  lower  part 

and  represents  an  overlying  flow.  They  are  dense,  fresh  rocks  with 

mmtm  ^*w  «•«•  iw<w^«M»«»^a»  mmw^^mm  ^M*IW^«M»  »••••  w»^i»«»^  "••  ««^  •»«••*•  ^•••••••••^  «»  ^*»mm^^  ^mm<^  «»  «  *•  *  •»^«»  «•  iv^^  mm  mm  mm  ^  «•»  •»*• 

3/  Ridgway»s  light  purple-drab  (l»'"b)  to  light  vinaceous  drab 
(5T»»»b),  rarely  cinnamon  drab  (13tftf-). 

i»i»«,^,»^.»^«.»...._.— •»—.•— p.- —•«•••  «»^-»"«""^"»  •••••• —^••••^^^"•^^•••'••»  •""•••  ••^^""•~^<~  ••*""•  *"^^ ""™"—" 

abundant  two  millimeter  crystals  of  white  plagioclase  and  a  smaller 
number  of  glassy  orghoclase,  quartz,  biotite,  green  augite,  and  black 

hornblende. 


Microscopic.-  An  examination  of  thin  sections  showed  that  in  texture 
the  rock  is  porphyritic  and  the  phenocrysts  nearly  equal  the  groundmass  in 
amount.   These  phenocrysts  are  up  to  three  millimeters  in  diameter  and  are 
plagioclase,  orthoclase,  quartz,  and  biotite  with  accessory  zircon,  iron 
ore,  titanite,  and  apatite.  The  plagioclase  crystals  range  from  andesine  to 
sodic  labradorite,  averaging  calcic  andesine;  it  is  commonly  largely  altered 
to  calcite,  less  commonly  to  sericite.  Some  of  the  larger  crystals  carry 
inclusions  of  small  biotite  plates  and  of  the  accessories.  The  biotite  has 
been  much  resorbed  by  the  magma  leaving  grains  of  iron  ore  along  the  border; 
it  is  also  commonly  altered  to  chlorite.  There  was  probably  originally  a 
considerable  amount  of  augite  or  hornblende  present  but  it  is  now  completely 
replaced  by  calcite  and  chlorite.  Secondary  calcite,  epidote,  and  chlorite 
are  rather  abundant  in  all  the  sections. 

The  large  part  of  the  groundmass  is  very  finely  crystalline  to  sub- 
microscopic  and  is  rhyolitic  in  character;  it  is  clouded  from  submicroscop- 
ic  inclusions  and  in  addition  is  dusted  with  minute  brownish  trichites. 
It  has  a  well-developed,  wavy  fluidal  texture.  Very  irregular  streaks  of 
the  groundmass  are  made  up  of  a  coarser  aggregate  of  quartz  and  orthoclase 
in  grains  or  patchy  micrographic  intergrowths.  In  texture  these  areas  are 
irregular  and  they  are  clear  except  for  scattered  trichites  of  opacite. 

The  rocks  of  the  upper  slopes  on  both  sides  of  West  Willow  and  Deerhom 
creeks  differ  from  the  above  in  the  greater  amount  of  plagioclase  and  less 
of  quartz  and  orthoclase;  the  original  biotite  is  almost  completely  resorbed, 
grains  of  augite  are  fairly  abundant,  and  partly  resorbed  prisms  of  brown 


or  green  hornblende  are  present.    The  groundmass  tends  to  be  spherulitic. 

Weathering  and  outcrops. 

The  rock  is  probably  even  more  resistant  to  weathering  than  the  Willow 
Creek  rhyolite  but  as  it  occurs  only  in  the  upper  glaciated  parts  of  the 
streams,  where  canyon  cutting  is  not  prominent,  it  forms  no  cliffs  or  can- 
yons comparable  to  those  of  the  Willow  Creek  rhyolite  near  Creede, 

New  the  Equity  Mine,  where  a  fault  separates  this  quartz  latite  from 
the  underlying  pillow  Creek  and  Campbell  Mountain  rhyolites,  there  is  a 
rather  striking  difference  between  the  outcrops  on  the  two  sides  of  the 
fault,  as  shown  in  Plate  XjftT  opposite  p.     The  Equity  quartz  latite 
on  the  right  shows  rather  rugged  outcrops  with  jagged,  broken  cliffs,  while 
the  rhyolites  form  steep  but  smooth  talus  covered  slopes.  Here  the  v7illow 
Creek  rhyolite  is  less  resistant  than  to  the  south  and  does  not  differ 
greatly  from  the  Campbell  Mountain  rhyolite*  It  yields  a  scant  soil  and 
at  the  base  of  the  broken  cliffs  develops  great  accumulations  of  slide  and 
talus.  The  physical  agencies,  change  in  temperature,  and  gravity  have  been 
most  important  in  the  breaking  down  of  the  rock* 


Chapter  4* 
Rocks  of  the  Upper  Division  of  the  Potosi  Volcanic  Series. 

General  Statement. 

The  rocks  which  are  here  included  in  the  upper  division  of  the  Potosi 
volcanic  series  are  separated  from  both  the  underlying  lower  division  and 
the  overlying  Creede  formation  and  Mac-^nzie  Mountain  quartz  latite  by  sur- 
faces of  makked  irregularlity.    Both  these  surfaces  are  due  to  erAsion 
during  periods  of  comparatively  little  igneous  activity.  Che  irregular 
character  of  these  surfaces  is  not  local  but  has  been  recognized  through- 
out the  eastern  part  of  the  San  Juan  Mountains. 

The  rocks  of  this  division  are  chiefly  rhyolites  and  quartz  latites  but 
in  about  the  middle  of  the  division  is  a  considerable  though  variable  thick- 
ness of  andesite.  The  rocks  beneath  the  andesite  are  chiefly  biotite  rhy- 
olites, and  latites,  those  above  are  closely  related  to  each  other  and  are 
hornblende-quartz  latites.  These  hornblende-quartz  latites  are  separated 
from  the  underlying  flows  by  a  somewhat  irregular  surface  of  erosion. 
Locally  the  base  of  the  tridymite  latite  is  also  very  irregular.  These 
erosional  surfaces  do  not  appear  to  have  been  so  extensive  or  so  irregular 
as  those  at  the  top  and  base  of  the  upper  division  of  the  Potosi. 

The  rocks  underlying  the  erosional  surface  at  the  base  of  the  quartz 
latites  differ  greatly  in  the  western  part  of  the  quadrangle  from  those 
occupying  the  same  position  in  the  eastern  part,  and  this  difference  itfl/ 
the  succession  of  flows  and  tuff  beds  for  different  parts  of  the  area 
together  with  the  lack  of  continuity  of  the  members  through  erosion,  cover, 
or  lack  of  deposition  has  made  the  correlations  and  separations  difficult 


and  in  some  cases  uncertain.  In  the  western  part  of  the  quadrangle  the  lowest 
formation  of  this  division  is  a  hornblende-  quartz  latite  made  up  of  a  chaotic 
aggregate  of  thin  flows  and  breccia  deposits.  It  is  overlaid  by  the  Willow' 
Gulch  rhyolite  breccia,  a  series  of  tuff  and  flow  breccia.  This  is  in  turn 
succeeded  by  a  thick  flow  of  tridymite  latite  which  ia  in  turn  succeeded  by 
the  andesite.  In  upper  'tfest  Willow  Creole  are  poor  exposures  of  a  quartz 
latite  which  differs  considerably  from  the  tridymite  latite  but  occupies 
about  the  same  horizon,  as  it  immediately  underlies  the  andesite.  In  the 
eastern  part  of  the  quadrangle  the  lowest  formation  of  the  upper  division 
of  the  Potosi  is  a  great  flow  of  the  Manmoth  Mountain  rhyolite,  a  flow 
breccia.  It  is  overlain  by  a  rhyolite  tuff  with  associated  thin  flows  of 
rhyolite  and  these  are,  in  turn,  overlain  by  the  tridymite  latite.  The 
andesine  is  absent. 

SS 

T  the  east  of  west  Willow  Creek  above  the  mouth  of  Deerhorn  Creek  the 

o 

section  is  still  somewhat  different,  although  exposures  are  too  poor  for  a 
positive  determination  of  the  relations  existing  between  the  rocks  of  this 
area.  The  tridymite  latite  appears  to  be  the  lowest  rock  exposed.  It  is 
overlain  by  50  feet  or  so  of  an  andesite  which  is  believed  to  be  the  same 
as  the  andesite  which  caps  Bulldog  Mountain.  This  is  overlain  by  100  feet 
or  so  of  thin-bedded  rhyolite  tuff  which  probably  corresponds  to  the  tuff 
which  overlies  the  andesite  to  the  west.  This,  in  turn,  is  succeeded,  on 
the  slopes  at  least  ,  by  a  thick  flow  of  rhyolite  which  has  not  been  recog- 
nized elsewhere  in  the  quadrangle  and  which  is  an  unusually  thick  flow  in 
the  quartz  latite  tuff.  Several  hundred  feet  of  chaotic  tuff  breccia 
overlies  this  flow. 


The  thickness  of  the  upper  division  of  the  PotosI  as  might  be  expected 
from  a  series  of  igneous  rooks  with  irregular  surfaces  at  both  the  top  and 
bottom  and  within  it,  varies  greatly  from  place  to  place.   Within  the 
Creede  Special  quadrangle  it  is  probably  greatest  to  the  east  of  Rat  Creek 
where  it  is  over  2,000  feet. 

Hornblende  quartz  latite. 
General  character  and  distribution. 

In  the  western  part  of  the  quadrangle  the  lowest  mapped  unit  included 
in  the  upper  division  is  a  rather  chaotic  aggregate  of  small  irregular  flows 
and  ciastic.  material.  The  rocks  vary  considerably  in  character  but  are 
chiefly  quartz  latites  with  conspicuous  hornblende  and  the  whole  mass  is 
therefore  called  a  hornblende-quartz  latite,  although  hornblende  andesite 
is  common  and  rhyolite  is  present.   This  rock  is  present  on  the  small  hill 
just  northeast  of  Monon  Hill.  A  more  extensive  and  continuous  but  thin 
layer  extends  westerly  nearly  on  the  contours  from  a  point  southwest  of 
Bulldog  Mountain  to  the  fault  near  the  Xreutzer  mine.   West  of  the  fault 
it  is  nearly  1,000  feet  higher  on  the  slopes  and  continues  to  the  quad- 
rangle line  and  farther  west  into  the  basin  of  Shallow  Creek,  but  has  not 
been  found  farther  west  and  appears  to  be  a  very  local  body. 

Thickness. 

As  this  latite  overlies  the  rocks  of  the  lower  division  of  the  potosi 
irregularly  and  as  its  top  is  fairly  regular,  its  thickness  varies  consid- 
erably. 7/lthin  the  Creede  Special  quadrangle  it  is  probably  nowhere  over 


200  feet,  but  higher  up  Miners  Creek  and  in  Shallow  Creek  it  is  considerably 
thicker. 

Petrography. 
Megascopic.-  In  color  the  rocks  vary  considerably;  the  greater  part 

TT 

are  drabs.   Most  of  the  fresh  rocks  are  rather  dense  and  nearly  all  show  a 

••^•"— —•——••••—••————••— —^-•——  —————————,•—.-_._«-..•.._______.._  •••  __».-._•—..___.._.._ ,^,^,,^^..^^m,,m^,mfmfmf, 

I/  Near  Ridgway»s  pale  quaker  drab  (l»ttffd)  or  purple-drab  (!»•««);  less 
frequently  olive-buff  (21f»fd),  or  a  related  color. 

more  or  less  prominent  fluidal  structure.  Hornblende,  which  is  the  most 
abundant  dark  mineral,  is  conspicuous  from  its  lustrous,  black  cleavage  faces; 
biotite  in  the  usual  black,  flexible  flakes  is  commonly  nearly  as  abundant, 
and  green  pyroxene  can  rarely  be  seen  with  a  pocket  lens.  White  plagioclase 
crystals  are  abundant  and  in  some  of  the  rocks  many  of  the  crystals  are  sev- 
eral millimeters  across.  Colorless  orthoclase  is  considerably  less  abundant 
and  in  many  of  the  rocks  it  is  wanting;  large  dull,  orthoclase  crystals  sev- 
eral centimeters  across  are  sparsely  present  in  some  of  the  rocks;  quartz 
in  visible  crystals  is  rarely  present.  The  groundmass,  which  is  about  equal 

<*3t 

in  amount  to  the  phenocrysta  and  is  holocrystalline,  can  4*~  ^e  resolved 
with  a  pocket  lens,  but  in  most  of  the  rocks  it  has  the  appearance  of  a 
rh;/olite;   less  often  of  an  andesite.       At  the  base  of  some  of  the  flows  there 
are%  a  few  feet  of  black  obsidian  carrying  the  usual  phenocrysts.     On  the 
whole  the  rocks  are  not  very  different  from  some  of  the  quartz  latites  of 
the  lower  division,  especially  those  of  Outlet  Tunnel  quartz  latite. 

Microscopic.-  The  study  of  this  sedions  showed  that  the  greater  part  of 
the  rocks  are  quartz  latites.       The  hornblende  is  commonly  brown  -  less 
conmonly  green;  both  varieties  are  present  in  distinct  crystals  in  some  thin 

sections.     It  h^usually  been  considerably  resorbed  by  the  magma;  the  biotite, 


has  teen  resorted  to  a  less  extent.  The  plagioolase  is  andesine  or  andesine- 
labradorite  with  a  more  sodic  border.  The  accessory  minerals  are  pleochroic 
apatite,  magnetite,  and  rare  titanite.   The  groundmass  varies  considerably; 
in  some  specimens  it  is  spherulitic,  in  others  microfelsitic,  microgranular, 
micropegmatitic  or  in  part  glassy;  it  is  made  up  chiefly  of  quartz  and  alkalic 
feldspar.  Secondary  chlorite,  calcite,  and  kaolinite  are  present. 

The  andesites,  which  are  not  abundant,  differ  from  the  quartz  latites 
chiefly  in  the  character  of  the  groundmass  which  in  the  former  is  made  up 
largely  of  minute  laths  of  oligoclase  feldspar  with  some  rods  and  grains  of 
pyroxene.  The  phenocrysts  are  somewhat  more  abundant  and  in  a  few  of  the 
rocks  those  of  feldspar  are  tabular.  The  rocks,  as  a  whole,  are  near  the 
border  between  the  quartz  latites  and  the  andesitea;  those  called  andesites 
do  not  differ  greatly  from  the  quartz  latites  and  there  is  a  fairly  con- 
tinuous gradation  between  the  two.   In  this  they  differ  considerably  from 
most  of  the  other  quartz  latites  of  the  quadrangle  which  are  considerably 
nearer  the  rhyolite. 

Locally  the  rocks  are  greatly  altered  by  processes  other  than  weather- 
ing. The  plagioclase  is  kaolinized,  the  hornblende  altered  to  chlorite, 
iron  oxide,  and  other  minerals,  and  the  rock  is  bleached  to  a  dirty  light- 
gray  or  white. 

Weathering  and  outcrops. 

The  breccia  portion  of  this  formation  breaks  up  readily  and  offers  few 
outcrops;  the  massive  rock  is  somewhat  more  resistant  but  it  too  is  a 
comparatively  soft  rock  and  rarely  gives  good  outcrops.  Within  the  quad- 
rangle this  quartz  latite  is  thin  and  it  has  not  }iad  a  great  influents  on 

the  development  of  the  topography. 


Windy  Gulch  rhyolite  breccia. 
General  character  and  distribution. 

A  rhyolite  breccia  made  up  of  light-colored  rhyolite,  in  part  an 
ordinary  tuff,  in  part  a  normal  flow  rock,  but  chiefly  a  breccia,  prob- 
ably a  flow  breccia  lies  beneath  the  tridymite  latite  in  the  drainage  of 
Windy  Gulch  and  to  the  west  and  is  here  called  the  Windy  Gulch  rhyolite 
breccia.  Its  porous  character  and  the  abundant  fragments  of  pumice 
which  it  carries  are  characteristic. 

It  overlies  a  fairly  regular  surface  of  the  hornblende-quartz  latite 
in  the  drainage  of  Rat  and  Miners  creeks  but  to  the  east  it  directly 
overlies  the  Campbell  Mountain  rhyolite.  Near  its  base  it  commonly 
carries  very  abundant  inclusions  of  the  underlying  rock.  It  is  every- 
where overlain  rather  irregularly  by  the  tridymite  latite.  To  the 
southeast  of  Bulldog  Mountain  it  is  in  contact  with  the  Creede  forma- 
tion which  is  below  it  on  the  slopes. 

It  forms  a  layer,  broken  in  places  by  faulting  or  by  a  quaternary 
covering,  from  west  of  MacKenzie  Mountain  to  the  Happy  Thought  Mine. 
Exposures  are  very  poor  and  the  lower  contact  in  particular  can  rarely 
be  mapped  with  accuracy.  The  body  BX  west  of  Mackenzie  Mountain  is 
poorly  exposed  and  its  lower  contact,  in  particular,  is  uncertain. 
On  the  east  side  of  the  Kreutzer  Fault  the  rhyolite  breccia  is  thrown 
down  nearly  1,000  feet;  the  apparent  thickness  here  is  probably  due 
to  a  disturbance  of  the  beds  near  the  fault  but  exposures  are  almost 
wanting.   East  of  the  fault  the  rock  continues  to  Rat  Creek  and  is 


<3 


nearly  horizontal;  beyond  Rat  Creek  it  rises  rapidly  and  crosses  ^/indy 
Gulch  just  below  Bachelor.   Beyond  Windy  (Julch  this  rhyolite  breccia, 
aside  from  a  small  outcrop  just  south  of  Bachelor,  is  mapped  only  as  a 
narrow  band  west  of  the  Amethyst  fault  and  between  the  Last  Chance  and 
Happy  Thought  mines.   This  is  in  an  area  almost  completely  covered  with 
a  mantle  of  debris  from  the  slopes  above  and  probably  also  of  ancient 
wash,  and  is  alnost  entirely  lacking  in  exposures.  Not  many  of  the 
numerous  prospect  shafts  can  be  entered,  but  their  dumps  furnish  suf- 
ficient data,  although  the  rocks  are  commonly  much  altered,  to  map  this 
area  with  a  fair  degree  of  accuracy.  A~if  is  probaol$  local  in  extent  as 


outside  the  Creede  Special  map  it  has  been  recognized  only  to  the  west, 
as  far  as  Shallow  Creek. 

This  rhyolite  is  commonly  from  100  to  200  feet  thick,  although  locally 
it  is  probably  much  thicker,  as  it  is  believed  to  have  spread  over  a  sur- 
face of  somex  relief.  However,  it  has  been  faulted  and  considerably  dis- 
turbed east  of  Miners  Creek  and  south  of  Bulldog  Mountain,  and  the  greater 
apparent  thicloiess  in  both  these  areas  is  in  part,  at  least,  due  to  imper- 
fectly understood  structure. 


,. 


Petrology. 

u 

Megascopic.-  In  color  this  rock  is  commonly  pale  red-brown;     less  often 

I/  Ridgway's  purplish  vinaceous  (lfttb)  to  pale  purplish  vinaceous  (lttff), 
brownish  vinaceous  (5tftb)  to  pale  brownish  vinaceous  (5ltff},  pale  vina- 
ceous (lftf),  pale  purple-drab  (ltltfd)  or  some  closely  related  color. 


it  has  one  of  the  lighter  shades  of  gray;  the  altered  rock  is  white  or  gray. 
The  luster  is  always  chalky  dull.  The  rock  is  made  up  in  considerable  part 
of  irregular,  ragged  fragments,  rarely  over  a  few  centimeters  across,  of 

a 

pumice  with  a  fibrous  appearance,  due  to  the  fin|L  elongated  pores.   These 
fragments  have  a  somewhat  lighter  color  than  the  body  of  the  rock;  in 
places  they  are  replaced  by  a  green  chloritic  material  or  by  drusy  quartz. 
They  weather  out,  leaving  very  numerous  rough  cavities  of  various  sizes. 
In  addition  there  are  fewer  fragments  commonly  not  over  a  centimeter 
across,  occasionally  over  a  decimeter,  of  a  variety  of  foreign  rocks;  these 
are  generally  much  altered  and  are  chiefly  of  the  lower  rhyolites  and  the 
hornblende-quartz  latite  but  are  occasionally  of  other  rhyolitic  and  ande- 
sitic  rocks.   The  matrix  is  always  less  porous  than  the  pumice  fragments 
and  in  some  of  the  rock  it  is  fairly  dense.  It  carries  a  few  millimeter 
crystals  of  glassy  orthoclase,  and  a  very  few  of  biotite.  Much  of  the 
rock  is  believed  to  be  a  flow  breccia,  although  a  part  may  represent  a 
"mud  flow"  or  tuff.  The  fragments  which  make  up  a  large  part  of  the  rocfc 
and  are  similar  to  the  matrix  except  for  their  greater  porosity  probably 
represent  fragments  that  were  shot  into  the  air  and  fell  back  into  the 
molten  magma.  These  flow  breccias  may  also  represent  material  that  was 


-. 


thrown  from  a  crater,  settled  about  the  vent  while  still  soft,  welded 
together,  and  flowed  much  as  any  viscous  lava. 

Microscopic,-  The  thin  sections  of  the  rock  show  a  few  crystals 
of  orthoclase  and  a  very  few  of  plagioclase,  biotite,  magnetite,  apatite, 
and  zircon,  in  a  highly  porous,  glassy  groundmass. 

Weathering  and  outcrops. 

The  Windy  Gulch  rhyolite  breccia  is  considerably  softer  than  the  over- 
lying tridymite  latite  or  even  than  the  Campbell  Mountain  rhyolite  which 
underlies  it  in  places.   It  forms  smooth  slopes,  grass-covered  or  timbered, 
with  rock  outcrops  only  in  very  favorable  places.  On  exposure  to  weather- 
ing the  pumice  fragments  are  removed  and  the  rock  breaks  up  into  honey- 
combed fragments  with  considerable  interstitial  fine  material.  Where 
erosion  is  not  too  rapid  a  soil  is  formed  in  which  are  few  rock  fragments 
and  these  chiefly  of  the  latitic  inclusions.  Smooth  slopes  are  characteristic. 

Comparison  with  Campbell  Mountain  rhyolite. 

The  typical  rocks  of  the  Campbell  Mountain  rhyolite  and  of  the  ^7indy 
Gulch  rhyolite  breccia  can  readily  be  distinguished.  They  differ  consid- 
erably in  color  and  much  of  the  latter  can  be  recognized  by  its  porosity 
and  especially  by  the  abundant  pumice  fragments  which  it  includes.  However, 
the  denser,  darker-colored  varieties  of  Windy  Gulch  rhyolite  breccia  are 
almost  identical  with  some  of  the  lighter  colored,  less  dense  varieties 
of  Campbell  Mountain  rhyolite.  On  weathering  both  lose  their  inclusions, 


become  highly  cavernous  and  are  still  more  difficult  to  distinguish. 
Areas  that  caused  especial  difficulty  in  the  mapping  are  on  the  ridge 
north  of  the  Corsair  Mine,  on  the  ridge  south  of  Bulldog  Mountain,  and 
northeast  of  Monon  Hill. 

Mammoth  Mountain  rhyolite. 
General  character  and  distribution. 

The  Mammoth  Mountain  rhyolite  is  a  single  thick  flow  of  rather 
uniform  character.  It  is  confined  to  the  northeastern  part  of  the 
Creede  area  and  to  the  mountains  to  the  east  and  northeast.  The  small, 
triangular  body  in  upper  Dry  Gulch,  east  of  Mammoth  Mountains,  is 
poorly  exposed;  it  is  probably  bounded  by  faults.  The  greatest  body 
is  to  the  north  of  this  and  directly  overlies  the  rocks  of  the  lower 
divisionof  the  Potosi  series.  A  considerable  body  is  on  the  ridge  between 
the  forks  of  pillow  Creek.  This  flow  has  not  been  recognized  to  the 
west,  south,  nor  north  of  the  Creede  Special  area  but  it  has  been 
followed  to  the  east  as  far  as  Bellows  Creek. 

Character  of  contacts  and  thickness. 

The  base  of  this  flow  is  very  irregular,  as  may  be  seen  from  an 
examination  of  Plate  II.  This  is  shown  to  the  east  of  Bast  Willow  Creek 
where  the  lower  contact  commonly  cuts  sharply  across  the  contours  and 
several  of  these  contacts  were  at  first  thought  to  be  faults.  It  gen- 
erally overlies  the  Phoenix  Park  quartz  Itatite  or  Campbell  Mountain  • 

rhyolite  but  locally  rests  on  the  Willow  Creek  rhyolite.  It  occupies 
about  the  same  position  in  the  section  as  the  hornblende-quartz  latite 
and  the  Windy  Gulch  rhyolite  breccia  to  the  west  but  its  relation  to 


these  rocks  could  not  be  determined.  Overlying  the  Mammoth  Mountain  rhy- 
olite  fairly  regularly  is  the  rhyolite  tuff  or  the  quartz  latite  tuff. 

East  of  Bast  Willow  Creek  this  flow  attains  a  thickness  of  a  thousand 
feet  "but  this  diminishes  rapidly  to  the  west  and  the  flow  wedges  out  in 
the  upper  drainage  of  Nelson  Greek.  It  is  believed  that  it  came  from 
the  northeast  or  east  and  that  it  did  not  extend  far  west  of  Campbell 
Mountain. 

Petrography. 

If 

Llegascopic.-  In  color  this  rock  is  characteristically  red-brown. 

I/  Ridgway's  russet-vinaceous  (9tft  -)j  rarely  a  darker  shade  and  very 
rarely  a  lighter  tint;  exceptional  specimens  contain  less  gray,  more 
oomrnon  ones  contain  more  gray  as  brownish  drab  (9fftt  -),  or  more 
orange  as  fawn-color  (13tff  -}. 

Its  luster  is  dull;  its  fracture  is  commonly  rough  and  tends  to  be 
hackly.  It  is  a  flow  breccia  and  is  decidedly  mottled,  though  somewhat 
less  strikingly  so  than  is  the  Campbell  fountain  rhyolite  which  it  very 
closely  resembles.  It  shows  a  fluddal  texture  only  on  close  examination, 

i, 

with  very  fini,  wavy,  streaks,  discontinuous  and  irregular.  The  rock 
is  fairly  dense  although  cavities  a  millimeter  or  so  across  are 
sparingly  present;  much  larger  cavities,  in  part  flattened,  are  excep- 
tional, but  are  rather  abundant  locally;  they  are  probably  confined 
to  the  base  and  topoof  the  flow*  They  are  in  part  filled  with  kaolin- 
it  ic  material.  At  the  base  of  the  flow  wherever  seen  there  are  a  few 
feet  of  black  glass* 

The  rock  contains  rather  abundant  phenocrysts  up  to  2  millimeters 
in  cross-section;  they  make  up  approximately  10  per  cent  of  the  rock. 
(Jlassy  orthoclase  is  in  slight  excess  over  porcelain  white  plagioclase; 


quartz  and  biotite  are  less  abundant.   The  groundmass  is  aphanitic. 
The  mottled  appearance  is  due  to  the  presence  of  abundant  inclusions  which 
are  of  two  kinds,  one  distinctly  foreign,  the  other  differing  from  the 
host  chiefly  in  their  slightly  paler  color.    The  former  are  commonly 
much  altered  and  are  chiefly  of  quartz  latites  similar  to  those  of  the 
underlying  series;  less  common  ones  are  rhyolites  similar  to  those  of  the 
older  series,  or  of  more  coarsely  porphyritic  rhyolites  or  of  andesites. 
In  some  cases  they  are  bounded  by  narrow  bands  of  lighter  or  darker  color. 

Microscopic.-  The  thin  sections  show  that  the  plagioclase  ±x  of  the 
phenocrysts  is  oligoclase  or  andesine;  zonal  growths  are  inconspicuous. 
The  biotite  is  slightly  resorbed.  Minute  to  submioroscopic  particles 

ii 

of  ferritic  material  give  the  groundmass  a  pinkish  buff  color  or  a 
I/  Ridgway's  pinkish  buff  (17»fd). 

paler  tint  as  seen  in  a  thin  section  of  normal  thickness.  The  groundraass 
is  not  markedly  fluidal  but  shows  irregular,  discontinuous,  wormlike 
streaks  of  slightly  different  color  and  texture.  It  is  largely  submicro- 
scopic  in  crystallization  but  in  part  is  delicately  spherulitic  with 
the  minute  fibers  arranged  normal  to  the  fluidal  streaks.  It  has  the 
characteristics  of  a  rhyolite  groundmass  and  is  no  doubt  made  up  chiefly 
of  quartz  and  orthoclase.  In  addition  to  the  streaks  mentioned  above, 
and  fcuch  larger,  some  several  millimeters  across,  are  irregular  lenses 
and  streaks  of  colorless  material.  These  are  much  more  coarsely  crys- 
talline than  the  body  of  the  groundmass  and  are  made  up  of  microscopic 
intergrowths  of  quartat  and  orthoclase;  they  commonly  show  a  concentra- 
tion of  quartz  in  their  centers.  Small  rounded  areas,  rarely  a  milli- 


meter  across,  are  abundant  in  some  of  the  sections  and  are  largely  con- 
fined to  the  coarsely  crystalline  areas.   They  are  made  up  of  a  mineral 
which  has  an  indea.  of  refraction  of  about  1.50  and  a  very  low  birefringence, 
and  is  probably  a  zeolite.  Some  of  the  original  cavities  are  filled  with 
a  fibrous  or  platy  kaolinitic  material  whictthas  an  index  of  refraction 
of  1.535  and  a  moderate  birefringence;  in  some  specimens  this  is  depos- 
ited on  the  spherulites  mentioned  above. 

In  addition  to  the  inclusions  of  quartz  latites  there  are  common 
inclusions  of  a  rhyolite  porphyry  with  phenocrysts  of  orthoclase,  plagi- 
oclase,  quartz,  and  sphene  in  a  fairly  coarse  granophyric  groundmass. 
Fragments  of  altered  andesite  are  rare.  Other  inclusions  of  rhyolite, 
differing  slightly  from  the  host  but  sharply  bounded  and  commonly 
having  a  more  coarsely  crystalline  groundmass  and  paler  shade  of  color, 
are  abundant.  Many  of  them  are  clearly  included  fragments  but  some 
can  hardly  be  distinguished  from  the  areas  of  coarser  crystallization 
which  are  believed  to  have  crystallized  in  their  present  position. 

Weathering  and  outcrops. 

This  rhyolite  is  a  rather  resistant  rock  and  has  weathered  largely 
through  mechanical  agents.  It  has  no  banding  or  regular  direction  of 
weakness  but  the  weathering  is  influenced  by  the  inclusions  and  the 
irregular,  streaked  structure,  causing  it  to  disintegrate  into  very 
irregular,  hackly  fragments  commonly  about  the  size  of  a  pea;  on  the 
steeper  slopes  where  the  loose  material  is  carried  away  as  rapidly  as 
formed  the  outcropping  rock  is  very  rough  and  hackly.  The  rock  is 


uniform  in  character;  structureless  for  a  thousand  feet  in  thickness, 
and  more  resistant  than  either  the  underlying  quartz  latite  or  the  over- 
lying tuff.   It  gives  rather  prominent  outcrops  rough  in  detail,  and 
steep,  fairly  regular  slopes  with  no  prominent  cliffs  or  benches,  it  is 
best  exposed  in  First  Fork  of  East  7/illow  Creek. 

Comparison  with  the  Campbell  Mountain  rhyolite. 

The  Mammoth  Mountain  rhyolite  is  easily  distinguished  from  all  the 
associated  rocks  except  the  Campbell  Mountain  rhyolite,  which  it  very 
closely  resembles.  Both  rocks  are  flow  breccias  and  contain  foreign 
inclusions  of  quartz  latite  and  other  rocks,  in  addition  to  those  dif- 
fering from  the  host  chiefly  in  their  paler  tone  of  color;  both  rocks 
are  rather  dense  with  few  small  gas  cavities  and  both  have  a  dull  luster; 
both  rocks  show  indistinct  and  poorly  developed  flow  structure.  On  the 
whole  they  differ  slightly  in  color  but  the  variation  in  either  ono  is 
greater  than  the  difference  between  the  two.   The  Campbell  Mountain 

rhyolite  is  commonly  a  little  duller  or  more  brownish  than  the  Mammoth 

I/ 
Mountain  rhyolite.   The  phenocrysta  are  about  the  same  in  both  rocks. 

•••.•9MW»«»MM»MM»*HM«~««HI»_M>»««»WOTWM  MMM..M.»MM«I l»»»«»«.«»^^«-*»».—  »~— »•—  —  « •  ^•••••-•^••••^  «•••  ^»»-«HW  « 

I/     The  Campbell  Mountain  rhyolite  is  commonly  purple-drab   (!»'"-) 
or  a  nearly  related  color,  while   the  Mammoth  Mountain  rhyolite  is  com- 
monly near  russet-vinaceous   (9ffl-). 

_____„„___•__—__—————————————————————————•-— —————-•———•-——— —————— ———•"———     — 

The  microscopic  examination  aided  but  little  in  their  distinction.  In 
brief,  the  difference  between  various  specimens  of  the  Campbell  Mountain 
rhyolite,  specially  between  the  drab  and  the  gray  types,  is  much  greater 
than  that  between  the  drab  type  of  the  Campbell  Mountain  rhyolite  and  the 


typical  Mammoth  Mountain  rhyolite  and  if  a  number  of  typical  hand  speci- 
mens of  each  were  mixed  together  they  could  not  be  separated  except  by  one 
who  was  thoroughly  familiar  with  both  rocks.  Only  after  a  carfeul  study 
of  both  types  and  of  numerous  specimens  from  each  has  the  author  been 
able  to  distinguish  between  the  two  with  reasonable  assurance. 

One  of  the  most  constant  and  characteristic  differences  between  the 
two  rocks  is  in  the  weathering.  The  Campbell  Mountain  rhyolite  weathers 
into  flat  flakes  with  smoother- rounded  surfaces  often  a  foot  or  so  across; 
it  breaks  with  a  smooth  conchoidal  fracture.  The  Mammoth  Mountain  rhyo- 
lite almost  invariably  weathers  into  small,  irregular  hackly  fragments, 
most  of  them  less  than  an  inch  across;  its  outcrops  nearly  everywhere  show 
this  character  of  the  weathering  and  specimens  broken  from  even  apparently 
fresh  rock  show  an  irregular,  hackly  fracture;  only  exceptionally  do  they 
show  a  smooth  conchoidal  fracture.   The  difference  in  color,  as  stated 
above,  is  not  in  itself  conclusive  but  aids  in  distinguishing  between 
the  two  rocks.  The  general  appearance  and  inclusions  of  the  two  rocks 
are  somewhat  different.  In  the  Mammoth  Mountain  rhyolite  the  fragments 
of  rhyolite  similar  to  the  host  except  in  color  are  less  conspicuous, 
hence  the  rock  is  less  prominently  mottled  than  is  the  Campbell  Mountain 
rhyolite.  Inclusions  of  porphyritic  rhyolite  nearly  white  in  color  are 
locally  characteristic  of  the  -ammoth  Mountain  rock.   The  phenocrysts, 
especially  those  of  biotite  and  plagioclase,  are  somewhat  more  abundant 
in  Mammoth  Mountain  rhyolite,  and  the  whole  groundmass  has  a  wavy, 
chaotic,  delicate  flow  banding.  In  the  thin  sections  a  chaotic,  discon- 

V 

tinuous  wort-like,  fluidal  banding  is  rather  characteristic  of  some  of 


fc>  / 


the  rocks  of  the  upper  division  of  the  Potosi.  The  black  glass  at  the 
base  of  the  Manmoth  Mountain  rhyolite,  and  the  cavernous  rock  which  is 
locally  present  in  it  aid  in  separating  it  from  the  underlying  rhyolite. 
The  presence  between  the  two  rocks  of  a  small  amount  of  pheonix  park 
quart2  latite,  even  though  not  in  sufficiently  definite  bodies  for 
mapping,  has  aided  in  separating  the  two. 

After  a  careful  second  examination  of  much  of  the  contact  west  of 
Sast  Willow  Creek  the  separation  has  been  made  with  more  confidence  and 
accuracy  than  was  at  first  believed  possible.  The  greatest  difficulty 
was  experienced  in  the  area  just  west  of  Phoenix  -^ark  where  exposures 
are  poor  and  much  of  the  rock  is  of  doubtful  character.  The  final  map- 
ping includes  in  the  Mammoth  Mountain  rhyolite  some  outcrops,  just 
south  of  the  small  stream  which  passes  through  Phoenix  park,  which  from 
the  specimens  alone  might  be  included  in  the  Campbell  Mountain  rhyolite, 
although  they  are  not  typical. 

Rhyolite  tuff. 

General  character  and  distribution. 

On  the  upper  slopes  to  the  east  of  Phoenix  Park  and  in  large  part 
beyond  the  boundary  of  the  quadrangle,  a  siliceous  tuff  with  associated 
thin  flows  of  rhyolite  breccia  rather  regularly  overlies  the  Manmoth 
Mountain  rhyolite  and  is  in  turn  overlain  regularly  by  the  tridymite 
latite.  It  is  about  200  feet  in  thickness  and  has  one,  locally  two, 
thin  flows  near  its  base.   The  main  part  is  a  sandy  tuff  which  in 
places  carries  scattered  pebbles;  it  is  very  poorly  sorted  and  bedded. 


lot 


Petrography. 

fI5ie  tuff  is  nearly  white  to  pale  drab  and  has  in  large  part  a  sandy 
texture.  It  carries  very  abundant  glassy  crystals  of  feldspar  and  black 
biotite  up  to  several  millimeters  across  and  a  few  small  fragments  of  a 
rock  which  carries  abundant  phenocrysts  in  a  pumiceous  matrix.  Larger 
pebbles  of  similar  rock  and  of  a  variety  of  other  rocks  are  locally 
present. 

A  part  of  the  material  is  a  quartz  latite  tuff  made  up  largely  of 
broken  crystals  of  plagioclase  with  considerable  orthoolase,  some  quartz, 
biotite,  green  hornblende,  and  augite  in  a  very  fine  glassy  matrix,  and 
closely  resembles  the  quartz  latite  tuff  under  Nelson  Mountain. 

The  flows  are  rarely  over  25  feet  thick  KKUL  and  where  two  are 
present  they  are  separated  by  a  small  amount  of  tuff.  The  rock  of  the 
flows  is  a  rather  porous,  light  red-brown  rhyolite  flow  breccia  with 
numerous  small  fragments  of  pumice  and  rhyolite  and  a  few  of  a  fine- 

l/  Ridgway's  light  russet-vinaceous  (9tft  b)« 

textured  granitic  rock.  It  carries  scattered  glassy  crystals  of  ortho- 
clase,  white  andesine,  and  black  biotite  with  accessory  apatite,  zircon, 
and  magnetite.  The  ^roundmass  is  largely  glass  with  numerous  trichites 
of  red  hematite  and  some  ropy  streaks  which  are  largely  crystalline. 


Tridymite  latite. 

General  character  and  occurrence. 

The  tridymite  latite  is  the  most  nearly  uniform  and  distinctly 
characterized  rock  of  the  upper  division  of  the  Potosi.  The  main  flow 
throughout  the  area  studied  is  a  banded,  fluidal  rock  with  lenses  or 
bands  which  are  highly  porous  and  are  filled  with  minute  drusy  crystals 
of  tridymite.  Hear  Bachelor  and  elsewhere  a  denser  rock  free  from 
tridymite  fnrms  the  base  of  this  formation  and  probably  represents  a 
different  flow.  The  tridymite  latite  is  confined  to  the  area  west  of 
the  Amethyst  fault.   The  main  body  forms  a  band  from  the  west  slopes 
of  LlacKenzie  Mountain  to  the  Happy  Thought  Mine;  it  is  commonly  nearly 
horizontal  but  is  displaced  by  several  faults  and  locally  near  the 
faults  dips  steeply.   Small  isolated  bodies  of  the  typical  rock  are 
present  about  half  a  mile  west  of  Sunnyside,  and  others  north  of  Monon 
Hill;  isolated  bodies  of  less  typical  rock  are  present  in  Upper  Rat 
Creek  and  northwest  of  the  mouth  of  Deerhorn  Creek.  The  presence  of  the 
bodies  north  of  Monon  Hill  and  west  of  Sunnyside  can  be  explained  only 
on  the  assumption  that  the  tridymite  latite  flowed  over  a  surface  of 
greater  relief  in  this  area  than  has  been  recognized  elsewhere.   The 
correlation  of  these  bodies  is  based  entirely  on  the  character  of  the 
rocks,  without  confirmation  from  the  sequence  of  flows.   Such  a  cor- 
relation is  rarely  beyond  question.  In  the  Creede  area,  however,  the 
tridymite  latite  is  a  unique  rock  and  has  been  found  at  only  one 
horizon. 


7° 


TO  the  west  of  the  area  included  on  the  map  the  tridymite  latite 
is  well  developed  under  Bristol  Head  where  its  upper  surface  forms  the 
great  flat  to  the  north  of  Bristol  Head;  to  the  east  it  underlies  the 
great  bench  called  Wason  Park.   A  similar  rock  which  is  probably  a 
part  of  the  same  flow  or  a  very  closely  related  flow  is  present  south 
of  the  Rio  (Jrande  in  the  drainage  of  Trout  Creek.  Nearly  everywhere  it 
immediately  underlies  the  latite  tuff  but  under  Bristol  Head  the  two 
are  separated  by  andesite.   The  details  of  the  distribution  within 
the  Creede  Special  quadrangle  are  shown  on  Plate  II. 

It  is  not  present  on  the  slopes  of  Nelson  Mountain,  although  it 
covers  great  areas  both  to  the  east  and  west.   It  was  probably 
locally  eroded  from  this  area  preceding  the  deposition  of  the  quartz 
latite  tuff  although  it  may  never  have  covered  the  area  where  the 
Nelson  Mountain  ridge  now  is. 

Thickness. 

The  thickness  of  this  latite  varies  greatly  and  rapidly  from  place 
to  place,  due  chiefly  to  the  irregularity  at  its  base.  Its  greatest 
thickness  is  about  400  feet  beneath  Bulldog  Mountain;  from  this  it 
decreases  to  50  feet  west  of  MacKenzie  Mountain. 

Petrography. 
Megascopic.-  In  color  the  rock  is  rather  dark  red-brown. 


I/  Ridgwayfs  vinaceous-brown  (5ttfi)  or  a  nearly  related  color;  the 
paler  tints,  deep  brownish  vinaceous  {5tlf-f  and  brownish  vinaceous  (S' 
are  common,  as  are  also  the  colors  with  wsLKxjpcxy.  more  red,  livid  brown 
(!»»»-)  and  purplish  vinaceous  (llflb)  and  those  with  more  gray,  purple 
drab  (!'"»-)  and  dark  purple -drab  (lfftfb);  paler  tints  and  darker 
shades  are  exceptional. 


It  is  characteristically  banded  and  platy;  the  main  part  is  rather  dense  but 
irregular  streaks  and  lenses  up  to  a  centimeter  across,  of  a  paler  tint,  are 
decidedly  porous.  These  are  rather  evenly  spaced  at  a  few  centimeters  apart, 
and  make  up  a  considerable  part  of  the  rock.   The  rock  shows  phenocrysts 
of  white  plagioclase,  glassy  orthoclase,  and  black  biotite  nearly  equal  in 
amount  to  the  groundmass  and  up  to  two  millimeters  across.   The  pores  are 
characteristically  lined  with  minute  drusy  crystals  of  tridyraite. 

Microscopic.-  The  microscopic  examination  showed  that  the  phenocrysts 
are  orthoclase  and  plagioclase  in  nearly  equal  amount,  considerable  biotite 
and  accessory  apatite,  iron  ore,  and  zircon.  The  plagioclase  has  a  core  of 
andesine  or  andesine-labradorite  grading  through  two  or  more  rather  broad, 
intermediate  zones  to  a  narrow  border  of  albite  or  oligoclase;  they  average 
about  andesine.   They  are  commonly  in  part  altered  to  sericite  and  kaolinite 
The  biotite  has  been  more  or  less  resorbed  by  the  magma  with  the  separation 
of  iron  oxide.  The  groundmass  is  beautifully  fluidal  with  wavy  bands.  The 
main  part  is  so  finely  crystalline  as  to  be  only  indistinctly  polarizing, 
it  is  clouded  and  reddish-brown  in  reflected  light  from  numerous  microscopic 
specks  of  hematite.   It  is  probably  made  up  largely  of  orthoclase  but  may 
have  some  quartz.   There  are  numerous  lenses  or  streaks  of  clear  material 
which  has  a  much  coarser  crystallization  and  the  larger  ones  are  porous. 
These  vary  in  width  from  a  fraction  of  a  millimeter  to  several  millimeters. 
They  vary  greatly  in  the  coarseness  of  their  crystallization  and  are  in 
part  spherulitic  or  fibrous,  in  part  micrographic  or  miorogranular.  Ortho- 
clase and  tridymite  are  the  chief  minerals  of  these  streaks.   Tridymite  is 


very  abundant  and  is  present  as  aggregates  filling  rounded  areas  or  lining 
the  walls  of  the  cavities  and  is  evidently  closely  associated  with  the  gas 
cavities.   In  a  few  specimens  tridymite  is  absent  and  its  place  in  the  cen- 
ters of  the  coarsely  crystalline  bands  is  taken  by  quartz.   In  one  specimen 
from  upper  Rat  Creek  the  cavities  are  lined  with  botryoidal  opal. 

The  origin  of  these  coarsely  crystalline,  porous  bands  is  believed  to 
have  been  much  the  same  as  that  of  the  somewhat  similar  bands  in  Willow 
Creek  rhyolite,  and  has  been  discussed  (p.   )  in  connection  with  the  de- 
scription of  that  rock.  However,  conditions  were  not  identical  as  in  this 
flow  a  large  part  of  the  silica  is  in  the  form  of  tridymite  and  the  amount 
of  space  occupied  by  the  gas  cavities  is  considerably  greater. 

Chemical.-  A  specimen  from  about  100  yards  northwest  of  the  school- 
house  at  Bachelor  was  selected  for  analysis  as  representing  the  typical 
tridymite-rich  rock.  The  material  for  analysis  was  taken  across  the  bands 
so  as  to  represent  the  average  of  the  flow.   The  plagioclase  is  slightly 
kaolinized  but  otherwise  the  rock  is  fresh.   An  analysis  was  made  by 
Gteorge  Steiger  in  the  Survey  laboratory  and  is  given  as  follows: 


Analysis   of  Tridymite  Latite, 


L  2 

67.76 

Ti02 

.45 

A12°3 

16.08 

Zr02 

.02 

F92°3 

2.22 

fl^\ 

W  W 

none 

FeO 

.23 

?  0 

.11 

2  5 

MgO 

.43 

S 

.02 

CaO 

2.59 

LtoO 

.04 

Na  0 

4.06 

BaO 

.12 

2 

K2° 

4.91 

SrO 

.03 

HO- 

.94 

f. 

H  0 

.54 

100.55 

The  norm  computed  according  to   the  quantitative  classification  is; 


Quartz 

19.78 

Hematite 

2.22 

Orthoclase 

28.91 

Ilraenite 

.61 

Alb  it  e 

34.06 

Titanite 

.39 

Anorthite 

11.40 

Apatite 

.34 

Hypersthene 

1.00 

H  0,  etc. 

1.48 

2 

Diopside 

.22 

100.41 

The  mode  differs  from  the  norm  chiefly  in  the  presence   in  the  former  of 
tridymite  instead  of  quartz  and  of  Motite   instead  of  pyroxene. 
The  rock  is  a  Roscanose    (1.4.2.3). 


The  flow  near  the  mouth  of  Deerhorn  Creek. 

On  both  sides  of  Test  77illow  Greek  just   above  the  mouth  of  Deerhorn  Ore 
are  a  few  small  exposures   of  a  quartz  latite,  differing  somewhat   from  the  no 
mal  tridymite   latite.       Similar  material  was   found  on  the  dump  of  the  Captiv 
Inca  shaft.      Its   base   is  nowhere  exposed  and  the  overlying  rocks   are  but  poo 
exposed  on  the  northeast   side   of  the  creek.        It   is   immediately  overlain  by 
andesite  resembling  the  andesite  of  Bulldog  Mountain  and   is  believed  to  be 
closely  related  to  the  tridyraite  latite  and  has  been  mapped  as  belonging  to 
body. 

I/ 

In  color  the  rock  is  near  quaker  drab.        The  fresh  rock  shows  a  rather 

I/    Ridgway's  quaker  drab   (lffftf   -),   light  quaker  drab   (lftftt  b),   or 
1 ight  purple-drab   ( 1  *  * f '  b ) . 

delicate,    inconspicuous,  wavy  fluidal  banding  which  is  more  conspicuous  on 
the  somewhat  altered  rock.     The  rock  is  rather  dense  and  shows   scattered 
crystals  approximately  3  millimeters   in  cross-section  of  white  plagioclase 
and  clear,  glassy  orthoclase,  and  a  very  few  of  biotite.      In  much  of  the 
rock  the  plagioclase   is  altered  to  kaolinite  and  sericite. 

The  thin  sections  show  that  the  rock  contains  phenocrysts   of  orthoclase 
and  plagioclase   in  about   equal  amounts  with  less   of  biotite  and  accessory, 
minute  crystals   of  apatite,   zircon,   and  magnetite.     The  groundmass   is   fluida 
and   is   in  part   spherulitic,    in  part  microfelsitic.     The  rock  differs  from 
the  typical  tridymite  latite  chiefly  in  that   it    lacks  the  prominent   tridy- 
mite-rich  lenses. 


7-*- 


Outcrops,  weathering,  and  alteration. 

The  tridymite  latite  is  considerably  more  resistant  than  the  Windy  (Julch 
rhyolite  breccia  which  commonly  underlies  it ,  and  is  somewhat  more  so  than  the 
overlying  andesite.  It  is  commonly  traversed  by  a  system  of  vertical  joints 
and  these,  together  with  the  horizontal  fluidal  structure,  determine  its  mode 
of  weathering.   It  breaks  into  small  plates  or  irregular  fragments,  with 
little  chemical  decomposition.   It  is  a  cliff -forming  member  and  as  it  is 
underlain  by  a  comparatively  weak  rock  several  large  landslides,  notably 
the  one  west  of  Bulldog  Mountain,  have  broken  from  its  cliffs. 

an;l  pyominontT^trorcT^fij^  clitf  against  the  snow-covered  - 

-p.  ^  , 


It  is  bleached  and  silicified  near  the  amethyst  vein,  although  the 
orthoclase  crystals  are  still  fresh.  Nearly  everywhere  the  plagioclases  show 
some  alteration  to  kaolinite  or  less  commonly  to  sericite,  and  locally  they 
are  completely  gone,  although  the  remainder  of  the  rock  shows  no  alteration. 
This  alteration:  probably  took  place  after  the  crystallization  but  before  the 
cooling  of  the  magma  and  was  probably  caused  by  the  gases  which  occupied  the 
pores  in  the  lava. 


Andesite. 
General  character  and  distribution. 

Andesites  and  related  rocks  occupy  only  a  very  small  part  of  the  quad- 
rangle. The  only  important  mass  of  rock  of  this  character  overlies  the 
tridyraite  latite  and  underlies  the  quartz  latite  tuff.  This  mass  is  made 
up  of  a  considerable  number  of  thin  flows  with  a  somewhat  smaller  amount 
of  intercalated  breccia.  The  rocks  vary  considerably  and  include  biotite- 
augite-quartz  laiites,  biotite-hornblende  andesites,  augite  andesites,  and 
olivine  andesites.  Normal  basalts  have  not  been  recognized. 

The  largest  area  occupied  by  these  rocks  is  on  Bulldog  Mountain  and  the 
ridge  to  the  north.  In  upper  Rat  Creek  are  two  small  outcrops  and  to  the  west 
of  Rat  Creek  there  is  a  narrow  band  of  this  andesite  above  the  tridymite 
latite.   In  west  T.7illow  Creek  just  above  and  below  the  mouth  of  Deerhorn 
Creek  are  two  small  outcrops  of  massive  rock  which  are  believed  to  belong 
to  this  mass.   Farther  east  this  rock  has  not  been  found.  To  the  west  of 
the  Creede  Special  quadrangle,  however,  on  Bristol  Head  these  andesites  are 
locally  well  developed  and  occupy  their  usual  position,  regularly  overlying 
the  tridyraite  latite  and  rather  irregularly  overlain  by  the  quartz  latite 
tuff. 

Throughout  the  area  over  which  this  andesite  has  been  studied  it  is 
irregular  in  thickness  and  discontinuous;  the  base  is  fairly  regular  and 
the  greater  part  of  the  variation  in  thickness  is  due  to  the  irregularity 
at  the  top.  The  greatest  thickness  is  north  of  Bulldog  Mountain  where 
there  is  about  500  feet  of  this  andesite  but  it  becomes  rapidly  thinner 
in  all  directions. 


77 


Petrography. 

I/ 
General.-   In  color  the  rocks  are   largely  near  quaker-drab  or  mouse  gray. 

— — - — — . _ _ _ __ _ — __ _ _ — 

I/  Ridgway's  quaker  drab  (!••»••  -)  or  deep  mouse  gray  (15'""  i)  or  a  lighter 

tone  or  darker  shade  of  either  of  these;  rarely  purple-drab   (!""   -) 

or  olive-gray  (23'""  b). 

•^» »*"^^— »^^« "^  — »  •_  «••••_  _  ._*  «.«•  «.—>•«•__••._•.»„._,,_,  ^+*m^  —  ••»*-•.  —  W_~*  ••__,  w^^  M  —  — •  _  ^  _»  _»  •*••»•»_—•»_»  _•—•_*.  n»  — ~  «•  _> 

The  greater  part  of  the  rocks  are  fine-textured  and  carry  very  few  crystals 
that  can  be  seen  without  a  careful  inspection  with  a  pocket  lens.   They  are 
as  a  rule  conspicuously  vesicular  or  amygdaloidal.  In  many,  the  vesicules 
are  much  flattened  by  flow,  giving  the  rock  a  platy,  f luidal  texture. 
Megascopically  they  were  thought  to  be  basalts.   The  lowest  flow  in  the 

«/ 

drainage  of   7indy  Gulch  is  dark  reddish  brown       and  rather  dense.      It   carries 
Zj     Hidgway •  s  dark  purple-drab   ( 1 "  "    i ) . 

scattered  crystals  a  few  millimeters  across   of  white  plagioclase,   and  brownish- 
black  biotite  and  hornblende   in  a  felsitic  groundraass.      On  the  east  bank  of 

\Vest    7illow  Creek  just   above  the  first  crossing  of  the  road  to  the  Equity 

5/ 

mine  is  a  small  exposure  of  a  dense,  deep  mouse-gray   rock  which  carries 

3/  Ridgway's  (15" '"  i). 

___—_«_____________—_—__-.——-.—-.--————————————— — — — — — _— _  _ _______ 

abundant  tabular  crystals  of  glassy,  striated  plagioclase  up  to  5  millimeters 
long  and  a  little  augite  in  an  aphanitic  groundmass.  A  similar  rock  was 

* 

brought   from  under  the  white   tuff  from  a  shallow  shaft     a  few  hundred  feet 
to  the  south  where  it   is  associated  with  the  normal  amygdaloidal  rock. 
Another  locality  ft)  r  this  type   is  from  the  bottom  of  a  shallow  shaft  west 
of   .Vest  Willow  Creek  at   an  elevation  of  10,900  and  S.   25°  U  of  the  mouth 
of  Deerhorn  Creek. 


Microscopic.-  As  seen  under  the  microscope  the  typical  amygdaloidal  rock 
contains  rather  abundant  millimeter  tablets  of  plajioclase,  with  a  little 
augite  and  altered  olivine  which  grade  into  a  very  fine-textured  groundmasa 
and  magnetite  with  considerable  brown,  clouded,  interstitial  glass.  A  small 
amount  of  quartz  is  present  in  some  of  the  rocks;  it  may  be  secondary,  although 
it  appears  to  be  interstitial  to  the  feldspars  of  the  groundmass.   -The  larger 
feldspar  crystals  have  cores  of  labradorite  or  sodic  labradorite  and  borders 
filled  with  glass  inclusions,  which  become;  as  sodic  as  albite.  The  feldspars 
of  the  groundmass  range  from  albite  to  andesine.  The  feldspars  average  about 
andesine.  The  olivine,  which  is  variable  in  amount,  is  completely  altered  to 
iddingsite,  fibrous  strongly  birefracting  serpentine,  carbonates  and  iron 
oxide.  The  augite  is  commonly  reddish-brown  from  the  partial  oxidation  of  the 
iron,  probably  before  the  consolidation  of  the  rock.  Some  of  the  sections 
show  areas  of  opacite  which  probably  represent  resorbed  crystals  of  amphibole 
or  other  dark  minerals.  A  few  of  the  rocks  are  holocrystalline  and  carry 
considerable  interstitial  albite  and  a  little  orthoclase.   Superficially 
the  rocks  resemble  basalts  but  differ  from  normal  basalts  in  the  more  sodic 
character  of  the  feldspar  and  the  small  amount  of  dark  minerals;  it  may  be 
called  an  olivine  andesite.   It  grades  into  a  normal  pyroxene  andesite. 
The  filling  of  the  vesicules  is  varied.  The  most  common  is  a  soft, 
dull,  opal-like  material  with  shrinkage  cracks  and  no  doubt  represents  a 
gelatinous  filling  now  partly  crystallized.   It  is  pale  yellow-green   or 

\J  Hidgway's  deep  sea-foam  green  (27ft  d). 

.•____  •  ••  —  ^^"•^^•"••^•"^"'•"* •  ««•"• •  *•  ^ ^«»«»  —  •»•» «•«» ^« 

a  related  color.   In  part  it  is  amorphous  and  in  part  it  is  made  up  of  fibers 
projecting  from  the  walls  of  the  cavities.  These  have  a  mean  index  of  re- 
fraction of  about  1.60,  a  rather  strong  birefringence  and  a  positive  elongation, 


It  is  probably  nontronite,  oeladonite,  or  a  related  mineral. 

A  very  few  minute  prisms  with  the  following  optical  properties  were 
found  in  this  material. 

Jj  =  1.62        Birefringence  -  .01  about  and 
2V    large       probably  optically  -  . 

On  one  face  it  shows  an  extinction  angle  (Z  A elongation)  of  24°  ,  On  the 
other  face  it  shows  sensibly  parallel  extinction  and  the  emergence  of  X  on 
the  edge  of  the  field.  It  is,  therefore,  probably  monoclinic.   It  is  faintly 
pleochroic  with  Z~pale  greenish-yellow,  Y-pale  orange,  and  X  —  pale  pinkish 
orange.   These  properties  do  not  determine  the  mineral. 

Calcite  is  a  rather  abundant  mineral  of  the  cavities  as  are  also  anal- 
cite  and  a  number  of  undetermined  zeolites  whose  optical  properties  are: 

Undetermined  zeolite  A»-   This  zeolite  occurs  in  spherulites  or 
radiating  prisms  with  the  following  optical  properties: 

V\~-  1.480,  Y -1.485,      27^- small  to  medium,  optically  -  , 
Z     elongation  •=•  about  43  .   It  is  very  abundant  in  spherulites  of 
radiating  prisms  attached  to  the  wall  in  the  cavities  of  one  of  the  ande- 
sites  and  is  the  only  mineral  in  the  cavities. 

Undetermined  zeolite  B.-  Sparsely  present  as  snow-white,  radiating, 
hair-like  fibers.  J)~  1.477,  birefringence  barely  perceptible,  elongation  -  , 
and  extinction  sensibly  parallel. 

Undetermined  zeolite  G«-  Aggregate  of  minute  shreds. 

!£-=-  1.51,  7f=  1.52,      elongation-*-. 

Undetermined  zeolite  D.-  Colorless  to  white  grains. 


^  1.505,  birefringence  ^.01  about,  2V  small,  optimally-)- ,  dispersiony< 
rather  strong,  extinction  sensibly  parallel. 


The  lowest  flow  in  Tlndy  0-ulch  is  a  blot  ite-hornblende  andesite.  It 
carries  rather  abundant  phenocrysts  of  plagioclase  and  less  of  brown  horn- 
blende and  biotite.  The  plagioclases  have  an  average  composition  of  sodic 
labradorite;  the  larger  crystals  carry  abundant  peripheral  inclusions  of 
glass.  The  hornblende  is  partly  or  completely  resorbed  by  the  magma,  leaving 
the  usual  skeleton  of  magnetite  grains  and  augite.  The  groundmass  carries 
abundant  laths  of  andesine  feldspar,  which  grade  into  the  larger  phenocrysts, 
in  a  very  fine  matrix  which  contains  rather  abundant  grains  of  magnetite  and 
augite  with  indeterminate  material  which  is  probably  largely  quartz  and 
orthoclase.  Tridyraite  is  rather  abundant  in  the  small  vesicules.  The  rock 
is  closely  related  to  the  quartz  latites. 

The  rock  outcropping  just  below  the  mouth  of  Deerhorn  Creek  on  the  east 
approach  of  the  bridge  across  7/est  Willow  Creek  and  in  the  two  prospects  to 
the  west,  is  a  Motite-pyroxene  andesite  or  quartz  latite.   It  carries 
rather  abundant,  large  phenocrysts  of  plagioclase,  augite,  resorbed  biotite, 
and  magnetite.   The  groundmass  is  made  up  of  plagioclase  laths  imbedded 
in  a  felted  to  raicropegmatitic  intergrowth  of  quartz  and  orthoclase,  specked 
with  iron  oxide.   Chlorit  ic  aggregates  were  probably  derived  from  small 

\y 

hfcpersthene  prisms.     The  plagioclase  phenocrysts  are  zoned  and  range  from 
calcic  to  sodio  andesine,   the  smaller  laths   of  the  groundmass  are  somewhat 
more  sodic.     The  rocks  are   intermediate  between  the  andesites  and  the  quartz 
latites. 


Alteration  and  Topography. 

The  alteration  of  the  olivine  to  iddingsite  and  the  deposition  of  the 
minerals  in  the  amygdules  are  believed  to  have  taken  place  while  the  lavas 
were  still  hot  and  decomposition  of  the  minerals  of  the  rook  by  weathering 
has  not  extended  to  any  considerable  depth.  The  breaking  up  of  the  rock 
has  been  largely  physical.  These  andesites  are  comparatively  soft  and  are 
in  thin  beds  but  are  much  more  resistant  than  the  overlying  tuff.  Its 
upper  surface  is  therefore  commonly  a  bench;  its  slopes  are  characterized 
by  a  succession  of  benches,  formed  by  the  successive  flows. 

Quartz  latite  tuff. 
General  character  and  occurrence. 

At  the  base  of  the  series  of  quartz  latites  which  overlies  the  andesite 
is  a  considerable  thickness  of  light-colored  tuff  «fcte  in  which  there  are 
locally  thin  flows.  The  greater  part  of  the  material  in  sandy  with  scattered 
pebbles  which  are  rarely  over  a  few  inches  across,  a  part  is  very  fine  grained 
and  a  very  little  is  conglomeratic.  The  coarser  material  is  thick-bedded 
but  some  of  the  finer  material  has  very  well-developed,  closely  spaced  lamina- 
tions. 

On  the  ridge  between  Deerhorn  anl  Meat  Willow  creeks  the  tuffs  carry 
a  larger  amount  of  How  rock,  the  tuff -breccia  is  more  chaotic  than  else- 
where, and  the  rocks  are  more  commonly  rhyolitic.  They  probably  represent 
a  lower,  local  horizon  deposited  in  a  valley.   The  base,  in  this  area,  is 
a  thinly  laminated  tuff,  not  over  a  hundred  feet  thick.   This  is  overlain 
by  an  irre-ular  flow  of  rhyrlite-latite,  locally  several  hundred  feet  thick, 
and  this  in  turn  is  overlain  by  a  chaotic  aggregate  of  thin  flows  and 


"- 


tuff -breccia.  On  the  west  slope  of  the  ridge  the  lower  part  of  this 
aggregate  is  a  glassy  flow  about  50  feet  4r  more  in  thickness  and  with  a 
highly  irregular  base.   The  extreme  northern  part  of  the  body  is  in 
part  tuff,  but  contains  a  considerable  amount  of  massive  rock  in  small 
irregular  bodies.   The  central  part  of  the  body  under  and  north  of 
the  point  with  an  elevation  of  11,406  is  in  large  part  white  tuff  and 
latite  breccia  with  subordinate  massive  roc*.  South  of  this  peak  and 
beginning  near  its  top  is  a  fan-shaped  body  of  massive  rock  with  the 
handle  of  the  fan  forming  the  narrow  ridge  just  below  the  peak  and  its 
broad  part  on  the  ridge  to  the  south,  where  it  directly  overlies  the 
rhyolite-latite.  This  fan-shaped  body  is  a  steeply  dipping  flow  which 
overlies  successively  the  tuff  and  rhyolite-latite.  Its  outline  is 
indicated  on  the  geologic  map  (Plate  II). 

The  quartz  latite  tuff  overlies  a  rather  irregular  surface  of  the 
andesite  to  the  north  of  Bulldog  Mountain.  Much  of  the  tuff  to  the  west 
of  ',7est  villow  Creek  is  covered  with  glacial  debris  and  landslide  and  even 
where  a  continuous  deposit  of  Quaternary  material  does  not  cover  the 
bedrock,  exposures  are  almost  entirely  wanting  and  the  mapping  is  neces- 
sarily unsatisfactory. 

Beneath  Nelson  Mountain  the  usual  tuff  overlies  in  part  the  rocks 
of  the  lower  division  of  the  potosi,  in  part  the  Mammoth  rhyolite.  Beyond 
the  quadrangle  to  the  north  and  west  this  tuff  is  very  prominent  and  with 
the  overlying  flows  of  quartz  latite  forms  prominent  cliffs  and  benches 
for  many  miles  to  the  north  of  Bristol  Head. 


Thickness. 

The  top  of  the  tuff  is  fairly  regular  but  the  base  is  locally  rather 
irregular.  Its  thickness,  therefore,  varies  considerably.  Locally,  it  is 
less  than  300  feet  in  thickness,  while  east  of  Nelson  fountain  it  is  over 
500  feet,  and  west  of  the  Park  Regent  mine  and  west  of  Deerhorn  Creek  it 
may  be  even  thicker* 

Petrography. 

a* 

The  tuff  is  commonly  nearly  white  with  a  drab  or  buff  c««t.  The 

finer  material  is  made  up  of  fragments  of  glass  with  some  broken  crystals 
of  quartz,  feldspar,  and  biotite.  The  sandy  part  contains  a  large  number 
of  millimeter  sized  crystals  in  a  finer  matrix  of  glass  fragments  with 
some  larger  fragments  of  felsitic  rock.  The  crystals  are  quartz,  ortho- 
clase,  plagioclase,  biotite,  hornblende,  and  the  usual  accessory  minerals. 
In  quantity,  size,  and  mineral  character  these  crystals  closely  resemble 
those  of  the  overlying  flows  with  which  the  tuff  as  a  whole  has  a  close 
similarity.  The  larger  fragments,  which  make  up  the  main  part  of  the 
tuff  only  on  the  ridge  west  of  Deerhorn  Creek,  are  chiefly  pumice,  but 
fragments  of  thin  platy,  fluidal  rhyolite  a»e  abundant  locally,  and  those 
of  other  rhyolites  and  quartz  latites  and  of  black  glass  are  present. 

Andesitic  rocks  are  rare. 

Massive  rook  in  the  tuff* 

Massive  flows  make  up  a  part  of  the  material  designated  as  quartz 
latite  tuff;  their  flow  and  extent  are  shown  approximately  on  plate  II. 
A  prominent  flow  is  on  the  slopes  east  of  Nelson  Mountain  and  extends  from 


the  eastern  landslide  to  and  beyond  the  quadrangle  line.     It   is  about  100 

feet  thick  and  is  just  below  the  11,000-foot  contour  line.       The  rock  is 

I/ 
quaker  drab     in  color.       It   is  dense  and  shows  scattered  crystals  of 

I/  Ridgway's  quaker  drab   (lffttf  -). 

......  _„„__«_-.—.———-———— -.—.-.———-"——————————— — — — .-.———— ————————— —————— 

glassy  orthoclase,  white  andesine,  and  golden  biotite  in  a  felsitio  ground- 
mass.  Under  the  microscope  the  groundmass  is  indistinctly  polarizing  and 
probably  contains  a  considerable  amount  of  glass.   Lenses  and  streaks 
of  the  groundmass  are  somewhat  coarser  in  crystallization  and  are  made  up 
of  orthoclase  crystals  in  a  matrix  with  very  low  index  of  refraction  and 
indistinct  birefringence.   It  may  be  secondary  opal.   A  few  crystals  of 
quartz,  brown  hornblende,  and  augite  are  present,  and  apatite,  zircon, 
and  magnetite  are  accessory.    It  is  quartz  latite  and  approaches  a  rhy- 
olite  in  composition. 

The  rock  of  the  flow  east  of  the  Amethyst  mine  has  rather  abundant 
kaolinized  plagioclase,  some  glassy  orthoclase,  considerable  pale  brown 
biotite,  a  little  embayed  quartz,  and  accessory  apatite,  zircon,  and  mag- 
netite.  The  groundmass  is  a  glass  with  incipient  crystallization.  Rather 
abundant  gas  cavities  carry  drusy  crystals  of  orthoclase,  quartz,  and   ^ 

tridymite. 

•me  greatest  flow  forms  the  slopes  on  both  sides  of  Tfest  billow 
Creek  above  the  month  of  Deerhorn  Creek.     The  rook  Is  light  drab       or  a 
nearly  related  color.       It   is  commonly  dense  and  sh^s  rather  abundant  crystals 

Z      Ridgway's  light  purple-drab   t1""'  ,, -  - 


up  to  3  millimeters  across  of  white  plagioclase,  and  a  few  of  glassy  ortho- 
clase  and  black  biotite  in  an  aphanitic  groundmass.   A  poorly  developed 
streaking  or  banding  is  commonly  present.  The  thin  sections  show  that  the 
phenocrysts  make  up  considerably  less  than  half  of  the  rock.  Plagioclase, 
having  the  composition  of  andesine,  is  the  most  abundant  of  these;  embayed 
quartz,  orthoclase,  partly  resorted  biotite,  yellow  titanite  in  large 
crystals,  and  the  usual  accessories  are  also  present.  In  some  specimens 
the  groundmass  is  very  finely  crystalline  and  is  largely  sphemlitic  with 
perhaps  some  glass.  In  another  section  it  is  made  up  chiefly  of  rather 
large,  irregular,  rounded  blotches  which  can  be  seen  only  in  polarized 
light  and  are  very  fine  intergrowths  of  quartz  and  orthoclase.   They 
are  imbedded  in  an  exceedingly  finely  crystalline  matrix  and  are  packed 
closely  together  in  parts  of  the  section  but  are  scattered  irregularly 
in  other  parts.   As  seen  in  ordinary  light  this  ground  has  a  rather 
prominent  fluidal  streaking  and  these  pass  through  the  polarizing  blotches 
and  the  finer  matrix  indiscriminately.  The  ground  is  in  all  cases  filled 
with  red  trichites  of  iron  oxide.  In  composition  the  rock  is  near  the 
border  between  the  rhyolites  and  the  quartz  latites  and  may  pro^arly 
be  called  rhyolite-latite.   This  rock  offers  little  resistance  to  weather- 
ing aad  disintegrates  into  small  hackly  fragments.  Outcrops  are  poor  and  most 
of  these  are  of  the  disintegrated  rock.  The  slopes  are  commonly  smooth, 
rather  steep,  and  show  the  disintegrated  rock  in  place  very  near  the 
surface. 


The  massive  rock  associated  with  the  tuff  which  overlies  this 
rhyolite-latite  west  of  Deerhorn  Creek  is  in  large  part  a  black  or  dark- 
gray  glass,  rarely  it  is  nearly  white.  It  carries  rather  abundant,  in- 
conspicuous crysta  s  of  glassy,  striated  feldspar  up  to  five  millimeters 
across,  and  some  biotite.  Microscopically  the  rocks  show  also  a  few 
crystals  of  orthoclase,  quartz,  and  titanite  with  accessory  zircon, 

apatite,  and  magnetite.  Many  of  the  rocks  have  also  green  hornblende 

\ 

and  augite.  The  titanite  is  abundant  in  millimeter-sized  crystals.  The 
plagioclase  is  zoned  andesine.  The  groundmass  of  the  glassy  rocks  shows 
incipient  crystallization,  chiefly  along  the  cracks.  In  some  specimens 
abundant  red,  rounded,  botryoidal  spherulites,  up  to  several  millimeters 
across  are  imbedded  in  a  greenish  glass*  These  flows  have  not  been 
separated  from  the  associated  tuff  on  the  geologic  map. 

The  rock  of  the  mapped,  fan-shaped  flow  south  of  the  peak  with  an 

i/ 

elevation  of  11,406  is  dark  purple-drab       and  superficially  resembles 
\J    Ridgway's  dark  purple  drab   (llttf   i). 

some  of  the  dense,  fine-textured  quartz  latites  of  the  upper  division  of 
the  Potosi.     It   carries  abundant,  millimeter-sized  crystals  of  andesine 
with  some  of  quartz,  orthoclase,  biotite,  augite,  and  titanite,  and 
accessory  apatite  and  magnetite.     The  groundmass  is  fluidal  in  part 
spheruliti? ,   and  in  part  very  finely  crystalline.     It  is  a  quartz  latite, 


feathering  and  outcrops. 

This  tuff  is  but  little  consolidated  and  is  a  relatively  soft  rock, 
being  much  less  resistant  than  the  underlying  or  overlying  rocks.   The 
overlying  quartz  latites  in  particular  are  thick,  cliff  -forming  flows  and 
the  soft  tuff  gives  way  under  their  load,  thttf  giving  rise  to  the  land- 
slides which  so  commonly  cover  the  tuff.  In  general  the  tuff  gives  com- 
paratively gentle  slopes  with  few  outcrops  and  there  is  commonly  a  more 
or  less  well-developed  bench  near  its  base.  -^ir^iiate^-Va,,  opposite  p. 
of  the  tuff  are  shown  to  the  rear  of  the  mines  from  the 


center  of  the  photograph  to  the  right.   On  the  extreme  right  the  gentle 
slopes  below  the  cliffs  are  formed  of  the  tuff;  modified  by  glaciation 
and  landslides.   Locally,  as  on  the  southeast  slopes  of  Nelson  Mountain, 
they  form  steep  slopes  with  good  outcrops.  These  white  outcrops  with 
their  striking  castellated  forms  are  locally  called  the  "white  elephants". 
Where  flows  are  present  in  the  tuff  they  form  a  succession  of  benches  and 
cliffs.  The  white  color  of  the  exposures  of  the  body  west  of  the  mouth 
of  Deerhorn  Creek  ^nd  the  extreme  irregularity  of  the  complex  has  led  the 
local  prospectors  to  call  the  mass  "The  Blowout". 


Rat  Creek  quartz  latite. 
General  character  and  distribution. 

Immediately  overlying  a  fairly  regular  surface  of  the  quartz  latite 
tuff  and  closely  related  to  it  is  a  series  of  lava  flows  with  some  inter- 
bedded  tuff,  here  called  the  Rat  Creek  quartz  latite  from  their  develop- 
ment in  Rat  Creek.  The  flows  are  mostly  thin  and  rather  irregular;  the 
tuff  forms  a  minor  part  of  the  material.  The  several  flows  and  the  inter- 
bedded  tuff  are  of  quartz  latites  of  very  similar  character;  they  differ 
but  little  from  the  material  making  up  the  underlying  tuff  and  from  the 
overlying  Nelson  Mountain  quartz  latite. 

This  latite  underlies  the  Nelson  Mountain  quartz  latite  on  the  ridge 
between  Rat  and  West  Willow  creeks,  but  owing  to  the  large  amount  of 
landslide  beneath  the  cliffs  of  the  Nelson  Mountain  quartz  latite,  it  is 
shovra  as  a  number  of  disconnected  outcrops.  It  is  also  present  on  the 
slopes  of  Nelson  Mountain. 

Thickness. 

Although  there  is  no  marked  irregularity  either  at  the  base  or  top 
of  the  Rat  Creek  quartz  latite,  it  varies  considerably  in  thickness. 
Between  Rat  and  West  Willow  creeks  it  is  only  150  feet  thick  at  the  southern 
exposures  but  becomes  considerably  thicker  to  the  north.  On  the  slopes  of 
Nelson  Mountain  it  is  from  400  to  500  feet  thick. 


Petrogrsp  hy. 

i/ 

Megascopic,-   In  color  the  rocks  are  pale  drab,     They  are  rather 

I/  Ridgway *s  pale  purple-drab  ( 1 • » » «  d ) ,  pale  quaker  drab  (1 f • » • *  d) f 
or  a  nearly  related  color. 


dense  and  have  fairly  well-developed  flow  structure  a&own  by  horizontal 
planes  of  fracture  but  not  by  prominent  banding,  Phenocrysts  a  millimeter 
or  two  in  cross-section  make  up  about  half  the  rocks.   These  are  chiefly 
white  to  moderately  clear,  striated  plagioclase;  there  is  a  less  amount 
of  clear,  glassy  quartz  and  orthoclase  and  considerable  biotite,  in  the 
usual  black  flakes,  and  hornblende  in  black  cleavable  prisms.  Light-green 
augite  is  abundant  in  most  of  the  rocks  and  yellow-brown  titanite  can  be 
seen  in  some.   The  groundmass  is  aphanitic.  Inclusions  of  quartz  latites 
and  rhyolites  much  like  the  body  of  the  rock  are  abundant  in  some  of  the 
flows. 

Microscopic.-  The  microscopic  examination  shows  that  the  phenocrysts 
are  somewhat  variable  in  size  and  are  commonly  broken.  Of  these,  plagio- 
clase is  the  chief,  embayed  quartz  and  orthoclase  are  never  abundant  and 
are  not  always  present,  biotite  is  always  present,  both  hornblende  and 
augite  are  commonly  present,  some  flows  contain  only  one  of  them;  sphene 
is  abundant  and  in  large  crystals  in  some  of  the  rocks,  magnetite,  apatite, 
and  zircon  are  accessory  and  tridymite  was  found  in  a  few  of  the  sections. 
The  plagioclases  show  the  usual  zonal  growths  and  vary  from  oligoclase  to 
oligoclase-andesine.   The  hornblende  in  nost  of  the  flows  is  a  chestnut- 
brown  variety,  but  in  some  a  green  or  olive-green  variety.  Both  it  and 


the  biotite  have  been  greatly  resorted  by  the  magma.  The  groundmass  is 
chiefly  spherulitic  intergrowths  of  quartz  and  orthoclase,  in  some 
specimens  it  is  partly  glassy.   Streaks  and  irregular  areas  of  coarser 
crystallization  are  common  in  most  of  the  specimens. 

Weathering,  outcrops,  and  topography. 

Most  of  the  Bat  Creole  rock  is  more  resistant  than  the  underlying  tuff 
but  less  so  than  the  overlying  flow.   Its  outcrops  are  rather  poor  and  it 
forms  rather  steep  slopes  between  the  gentle  slopes  of  the  tuff  and  the 
cliff  of  the  overlying  flow.   Much  of  it  is  covered  by  landslide. 

Nelson  Mountain  quartz  latite. 

General  character  and  distribution. 

The  upper  flow  of  the  upper  division  of  the  Potosi  on  the  Creede 
Special  quadrangle  is  a  regular  flow  of  quartz  latite  of  uniform  character. 
It  makes  the  nearly  continuous  upper  cliff  and  caps  the  divide  east  of  Rat 
Greek  in  the  northeast  corner  of  the  quadrangle,  it  is  also  the  cap-rock 
onpelson  Mountain,  whence  its  name,  the  Nelson  Mountain  quartz  latite. 

A  similar  flow  overlies  the  tuff  and  forms  the  mesa  north  of  Bristol 
Head.  It  commonly  forms  prominent  mesas,  nearly  surrounded  b  y  cliffs. 

Thickness. 

The  top  of  this  flow,  no  doubt,  corresponds  approximately  to  the 
comparatively  flat  tops  of  the  ridges.  It  is  fairly  uniform  in  thdckness 
and  between  Rat  and  West  Willow  creeks  it  is  probably  350  feet  thick; 
on  Nelson  Mountain  it  is  less  than  200  feet  thick. 


Petrography. 

if 

Megascopic*-  In  color  the  rock  is  rather  uniformly  red-brown. 

\J  Ridgway's  purple-drab  (lflfl  -}  or  a  slightly  lighter  tint  on 
darker  shade. 

The  phenocrysts  are  about  a  millimeter  in  cross-section  and  in  amount  about 
equal  the  groundmass.  They  are  chiefly  of  porcelain-white,  striated  plagio- 
clase,  with  rather  abundant  black  plates  of  biotite,  prisms  of  black  horn- 
blende, and  of  pale-green  augite;  glassy  orthoclase  and  quartz  are  less 
conspicuous.  The  groundmass  is  aphanitic  and  rather  dense;  fluidal  struc- 
ture is  present  but  not  conspicuous. 

Microscopic.-  A  detailed  microscopic  study  showed  that  the  plagio- 
clases  have  an  average  composition  of  andesine.   The  hornblende  is  of  a 
brown  variety  and  both  it  and  the  biotite  have  been  somewhat  resorbed  by 
the  magma.  The  quartz  phenocrysts  are  in  small  amount  and  are  embayed. 
The  accessories  are  titanite,  in  rather  large  and  abundant  crystals,  mag- 
netite, apatite,  zircon,  and  hematite.  The  latter  is  in  very  minute  specks 
and  shreds  as  a  pigment  to  the  groundmass.  The  groundmass  is  very  finely 
crystalline  and  is  largely  spherulitic;  it  is  made  up  chiefly  of  ortho- 
clase and  quartz.  Streaks  and  bunches  are  of  coarser  crystallization;  a 
little  tridymite  is  present  in  some  of  these  coarser  streaks.   The  rook 
is  not  very  different  from  some  of  the  quartz  latites  of  the  lower  div- 
i  ision  of  the  Potosi. 


Weathering  and  outcrops. 

The  minerals  of  this  rock  show  little  alteration  and  the  weathering 
is  largely  mechanical.        It  breaks   into  flat  plates  along  the  fluidal 

planes  and  on  the  tops  of  the  ridges  yields  a  fair  amount   of  soil.       As 

a 
it   is/very  resistant  and  rather  thick  flow  and  overlies  softer  tuffs  and 

thin  lava  flows,   it  breaks  down  largely  by  landslides   of  various  sizes, 
thus  causing  a  recession  of  the  cliffs  of  this  flow.        The  flow  forms 
mesas  at  its  upper  surface,   surrounded  by  almost  continuous  cliffs  below 
which  are  commonly  talus  heaps  and  landslide  debris.        ^JEke-^aasas— oM 

the  skyline^  on  -the  "left 


of 

gfrmam  against  the  skvllttft  on  thfr-eactreme  right  of  rlate  Ya 


ppooito  p. 


Chapt  er  V. 
Creede  Formation. 

Name,  General  Character  and  Occurrence, 

A  considerable  thickness  of  rhyolitic  tuffs  and  bedded  breccia,  which 
contain  local  bodies  of  travertinous  spring  deposits  in  their  lower  part, 
and  some  intercalated  lava  flows  in  their  upper  part,  accumulated  in  a 
deep  valley  or  basin  which  had  much  the  same  character  and  position  as  the 
present  valley  of  the  Rio  Grande  from  Wagon  vtfheel  Gap  westward  to  Trout 
Creer:,  a  distance  of  about  twenty-five  miles.  Its  width  is  considerably 
less.   The  name  Greede  formation  is  here  proposed  for  this  deposit  from 
its  extensive  and  characteristic  development  on  the  slopes  on  either  side 
of  tfillow  CreeK  about  the  town  of  Creede. 

I/ 

In  general  the  Creede  deposits  occupy  an  area  in  which  the  Hayden 

JL/  U.  S.  Geol.  and  Geog.  Survey  Terr.  Colorado  Atlas  Sheets  XV  and  XVII, 
Hayden,  1881. 

map  represented  Green  P.iver  Eocene  beds  with  Carboniferous  limestone  above 

2/ 

them  on  the  slopes.   The  Hayden   reports  contain  practically  no  reference 

j, , 

Zj  U.  S.  Geol.  and  Geog.  Survey  Terr.  Colorado  Nlrfkth  Ann.  Kept.  pp.  153- 

157,  Hayden,  1877. 

_______  ___  ____—____—_—_—_———_———————————— ....__.._.-___-...——— ___-.——— _—_——— _—-._—— 

to  these  deposits  in  the  Rio  Grande  Valley  and  it  seems  plain  that  the 
Creede  tuffs  in  the  bottom  of  the  valley  were  called  Green  River  Eocene, 
while  the  interbedded  travertine  deposits  which  exhibit  massive  outcrops 
in  some  places  were  referred  to  the  Carboniferous  without  any  good  ground. 


ft- 


Collections  of  plant  remains  from  the  lower  tuff  beds  of  the  Creede  formation 
correlate  these  beds  with  the  Florissant  lake  beds  (Miocene). 

The  character  of  the  basin  in  which  the  Creede  formation  was  deposited 
is  well  shown  by  the  way  in  which  the  beds  commonly  lap  up  on  the  steep 
slopes  of  the  older  basin  of  deposition  made  of  the  same  rocks  which  form 
the  abrupt  slopes  of  today.   This  is  also  shown  by  the  geologic  map 
(Plate  II).  The  Creede  beds  occupy  the  floor  of  the  valley  of  the  Rio 
Grande  and  are  surrounded  by  steep  slopes  of  the  older  rocks  which  commonly 
rise  three  thousand  feet  or  more  above  the  valley,  in  a  distance  of  a  few 
miles.   This  is  not  due  to  folding  or  faulting  but  to  the  fact  that  the 
Creede  formation  was  deposited  in  a  basin  deeper  than  that  of  the  present 
Rio  Grande  Valley  and  probably  with  steeper  walls.   The  irregularity  at 
the  base  of  the  Creede  formation,  resulting  from  this  is  well  shown  on 
both  sides  of  Creede,  to  the  east  the  base  of  the  Creede  rises  nearly 
two  thousand  feet  in  elevation  in  a  distance  of  less  than  a  mile  (Plate  II). 
In  PlateJJJ" opposite  p.  the  base  of  the  Creede  formation  is  at  the  top 
of  the  cliffs  on  the  right  of  the  picture  over  the  buildings. 

Structure  and  thickness. 

In  general  the  tuff  beds  dip  gently  away  from  the  mountains  toward 
the  south  and  southeast,  and  the  dips  are  commonly  steeper  near  the  borders 
of  the  body.  Dips  of  ten  or  even  twenty  degrees  are  common  near  the  bor- 
ders and  local  dips  are  much  steeper.   Away  from  the  borders  the  dips 
are  small,  but  accurate  estimates  of  the  average  dip  for  any  considerable 
-  area  are  difficult  on  account  of  the  poor  exposures  and  local  undulations. 


The  general  structure  in  the  tuffs  of  the  valley  is  that  of  a  gentle  syncline, 
It  is  not  known  how  much  of  the  dip  is  due  to  tilting  since  deposition,  as 
beds  deposited  rather  rapidly  in  a  comparatively  small  basin  surrounded  by 
very  steep  slopes  might  have  a  considerable  inclination  at  the  time  of  deposi- 
tion and  this  inclination  would  probably  be  steeper  near  the  borders  of  the 
basin. 

No  satisfactory  estimate  of  the  maximum  thickness  of  the  Creede  beds  can 
be  made,  as  the  structures  are  uncertain,  the  top  is  nowhere  preserved  and 
the  base  is  exposed  only  on  the  steep  sides  of  the  basin.   About  five  hun- 
dred feet  of  the  lower  member  are  exposed  on  both  sides  of  V7illow  Creek 
below  Creede  and  neither  the  top  nor  the  base  is  shown;  just  east  of  Creede 
where  the  top  of  the  lower  member  is  preserved  about  eight  hundred  feet 
are  exposed.  A  proximately  a  thousand  feet  of  the  upper  member  are  exposed 

T) 

east  of  '7indy  Gulch.   In  the  middle  of  the  valley  the  thickness  was  prob- 
ably considerably  greater  as  the  slopes  of  the  basin  of  deposition  are 
steep  wherever  observed. 

Subdivisions. 

The  Creede  formation  has  been  subdivided  On  the  map  into  three  lith- 
ologic  units  which,  however,  have  no  sharp  lines  of  separation.  The  lower 
member  is  made  up  entirely  of  fragmental  material,  the  greater  part  of  which 
was  deposited  by  water,  although  some  represent  talus  and  similar  accumula- 
tions. Much  of  this  material  is  thinly  laminated,  white,shaly  tuff;  part  is 
sandy  and  part  is  breccia  and  conglomerate.   The  material  is  entirely  of 
igneous  origin  and  a  greater  part  of  the  fragments  was  derived  from  the 


rocks  of  the  lower  division  of  the  potosi  volcanic  series. 

Interbedded  with  the  tuffs  of  the  lower  member  are  beds  of  travertine, 
which  was  deposited  by  springs,  probably  hot  springs,  at  the  time  the  tuff 
was  being  laid  down  and  at  various  horizons*   It  is  in  p  art  a  surface  deposit; 
in  part  a  deposit  in  the  channels  through  which  the  waters  came,  and  in  part 
a  cementation  and  replacement  of  the  tuffs.   It  occurs  in  very  irregular 
bodies  whose  contacts  are  not  always  sharply  marked.  Only  the  larger  bodies 
are  shown  on  the  map  and  their  boundaries  are  more  or  less  diagrammatic. 

The  material  of  the  upper  member  is  somewhat  coarser  than  that  of  the 
lower  member;  it  is  mainly  rather  well  bedded  breccia,  conglomerate,  and 
tuff.  A  considerable  part  of  the  fragments  is  of  a  rhyolite  found  only  here 
and  in  the  associated  thin,  intercalated  flows. 

The  Lower  Member. 

general  character.-  The  lower  member  of  the  Creede  formation  is  made 
up  in  large  part  of  fine  grained  and  well  bedded  material  which  is  commonly 
creamy  or  light  grayish.   In  part  it  is  very  thin-bedded,  shaly,  and  closely 
resembles  a  siliceous  shale,  although  composed  almost  wholly  of  volcanic 
material.  Much  of  this  tuff  carries  plant  remains,  some  of  them  well  pre- 
served   Thicker  beds  of  sandy  materials  and  beds  of  lenses  of  conglomerate 
are  present.  Most  of  the  pebbles  are  well  rounded  fragments  of  igneous 
rock,  though  some  are  subangular  or  even  angular.  Near  the  contacts  with 
the  underlying  rocks  coarse  and  angular  material  is  much  more  abundant,  and 
at  the  base  of  the  original  steep  slopes  of  the  older  rooks  the  local  material 
of  the  Creede  beds  really  represents  an  indurated  talus  from  the  cliffs, 
in  the  upper  part  of  the  member,  west  of  Rat  Creel?:,  the  material  is  more 


thickly  bedded  and  much  of  it  is  sandy  or  conglomeratic* 

petrography.-  The  shaly  tuff  is  rather  uniform  in  character  and  is  made 
up  in  large  part  of  fragments  of  rhyolitic  glass  with  an  occasional  fragment 
of  feldspar,  quartz,  and  biotite.  The  sandy  beds  carry  very  abundant  crystals 
of  orthoclase,  plagioclase,  quartz,  and  biotite,  rare  fragments  of  other 
minerals,  and  a  varying  number  of  fragments  of  felsitic  or  pumiceous  rhyolite. 

The  pebbles  of  the  conglomeratic  part  of  the  tuff  consist  chiefly  of 
fragments  of  the  underlying  rocks,  but  include  also  a  great  variety  of  other 
rocks,  chiefly  rhyolites  and  quartz  latites.  The  fragments  of  the  conglomerates 
and  breccias  about  Creede  were  chiefly  derived  from  the  Willow  Creek  and 
Campbell  Mountain  rhyolites  with  some  from  rocks  similar  to  the  quartz 
latites  of  the  lower  division  of  the  Potosi,  Large  angular  blocks  of  a 
tridyrnite  latite  similar  to  that  of  the  upper  division  of  the  Potosi  are 
present  just  east  of  the  town  of  Creede,    The  nearest  exposures  of  the 
tridymite  latite  are  much  higher  on  the  slopes  on  Bulldog  Mountain  or  in 
the  high  mountains  a  few  miles  to  the  northeast.   The  fragments  may  well 
represent  landslide  or  talus  blocks  from  cliffs  of  this  rock  that  have  since 
been  removed  by  erosion.  A  few  fragments  of  a  variety  of  other  rhyolites, 
quartz  latites,  and  andesites  that  can  not  be  correlated  with  any  of  the  rocks 
of-  the  vicinity  are  also  present  and  these  are  more  abundant  in  the  thin 
lenses  of  conglomerate  that  are  in  the  thin  bedded  tuffs  at  some  distance 
from  the  jnountains.  In  other  places,  as  near  Wagon  Wheel  Sap,  the  fragments 
in  the  tuff  are  chiefly  quartz  latites  similar  to  those  that  formed  the 
adjacent  slopes  at  the  time  of  deposition.  Where  the  fragments  are  angular 
and  especially  near  the  contacts,  the  greater  part  is  identifiable  as  be- 
longing to  the  rocks  immediately  adjacent,  in  the  Iowa  tunnel  and  elsewhere 


near  the  contact  with  the  lower  rocks  the  material  is  made  up  of  small 
angular  fragments  of  the  Willow  Creek  rhyolite  somewhat  indurated  and, 
no  doubt,  represents  simply  an  accumulation  of  talus  at  the  base  of  the 
steep  slopes  of  the  rhyolites  during  the  Creede  epoch. 

In  general  both  the  tuff  and  conglomerate  are  fairly  well  indurated 
and  are  much  harder  than  the  somewhat  similar  tuff  of  the  upper  division 
of  the  Potosi.  Near  the  travertine  bodies  it  is  filled  with  calcite  and 
much  of  the  coarser  material  is  cemented  by  reddish  brown  or  yellowish 
hydrated  iron  oxide  or  by  a  green  ferruginous  material.  Locally,  especially 
in  the  upper  part,  the  tuff  and  breccia  are  cemented  by  silica. 

Interbedded  travertine. 

Character  and  distribution.-  Within  tho  tuff  of  the  lower  member  of 
the  Creede  formation  are  a  large  number  of  bodies  of  travertine.  The  largest 
of  these  is  just  east  of  Sunnyside  Creek  where  it  forms  prominent  cliffs; 
other  bodies  are  mapped  between  Creede  and  Sunnyside  and  between  Creede  and 
-Dry  Gulch.   These  travertine  bodies  are  also  characteristic  accompaniments 
of  the  Creede  formation  beyond  the  Creede  and  Vicinity  quadrangle.  Large 
bodies  are  present  in  lower  Lime  Creek  near  the  Western  extremity  of  the 
Creede  formation,  between  Fir  and  Shallow  creeks  in  the  drainage  of  Farmers 
Creek,  near  Wagon  Wheel  Gap,  and  in  the  drainage  of  Deep  Creek.  They  are 
rather  more  abundant  near  the  borders  of  the  Creede  formation  than  in  the 
c  nter.  The  bodies  are  very  irregular  in  form  and  their  contacts  are  often 
indefinite;  consequently  the  mapping  is  much  generalized  and  is  intended 
to  show  only  the  approximate  form  and  location  of  the  larger  bodies.  A 
great  part  of  the  travertine  is  a  light  grayish  deposit  of  very  fine-grained 
calcite,  in  some  places  very  dense,  in  others  rather  porous  or  highly 


cellular.   There  is  commonly  a  considerable  amount  of  limonite  especially 
in  the  more  porous  variety  and  some  of  the  material  is  highly  ferruginous. 
Locally,  as  in  parts  of  the  body  north  of  the  hill  with  an  elevation  of 
920o  feet  to  the  west  of  Willow  Creek,  there  is  a  light,  porous  deposit  of 
gypsum,  stained  and  coated  with  limonitic  material.  Part  of  the  travertine 
encloses  fragments  of  rock.  Locally,  it  carries  much  chalcedony  with  some 
quartz,  filling  original  cavities  or  as  ve inlets,  and  in  places  it  contains 
deposits  of  siliceous  sinter. 

Origin.-   The  travertine  and  less  common  siliceous  sinter  were 
deposited  about  the  openings  of  springs,  probably  hot  springs,  which  must 
have  been  very  abundant  about  the  borders  of  the  old  basin.  The  travertine 
is  usually  mixed  with  or  interbedded  with  the  fine-grained  tuff,  and  in  some 
places  it  appears  to  act  as  a  cement  for  the  tuff;  in  part  it  forms  lenses 
or  less  regular  bodies  in  the  tuff.  Some  of  the  travertine  was  probably 
deposited  in  the  conduits  of  the  springs;  the  strip  through  the  middle  of 
the  body  east  of  Sunnyside,  which  forms  the  pronounced  outcrop  of  this 
body,  is  believed  to  have  been  so  deposited.   It  forms  a  dike-like  outcrop 
striking  a  little  north  of  east  above  the  9100  foot  contour  with  very  steep 
walls,  over  a  hundred  feet  high  in  places  on  the  southern  side,  and  a 
bench  or  low  depression  on  its  northern  side.  It  is  almost  entirely  traver- 
tine while  the  rest  of  this  area,  included  on  the  map  in  the  travertine, 
carries  much  tuff  intimately  mixed  with  the  travertine. 

Travertine  deposits,  somewhat  similar  to  those  in  the  Creede  formation, 
have  been  formed  elsewhere  on  a  large  scale  about  the  borders  of  interior, 
undrained  lakes,  from  the  precipitation  of  calcium  carbonate  from  the  water 
of  the  lake.   The  abundance  of  silica  and  iron  in  the  travertine  of  the 


/ 


Creede  formation  and  the  presence  of  bodies  of  cellular  gypsum  and  limonite, 
together  with  the  small  size,  irregular  form,  and  distribution  of  the  bodies 
make  it  seem  unlikely  that  this  travertine  was  so  deposited. 

The  Upper  member* 

general  character  and  distribution.-  The  upper  member  of  the  Creede 
formation  is  made  up  of  considerably  coarser  material  than  is  the  lower  member 
and  the  material  is  better  sorted,  better  bedded,  and  more  rounded  than  is  the 
talus-like  material  which  makes  up  most  of  the  coarse  part  of  the  lower  member. 
In  addition  to  fragments  of  the  older  rocks  from  the  nearby  slopes  it  carries 
abundant  fragments  of  rhyolites  found  only  in  this  member  and  some  of  the 
conglomerates  are  made  up  entirely  of  such  rhyolites;  thin  flows  of  similar 
rhyolites  are  interbedded  with  these  conglomerates. 

general  character  and  distribution.-  The  two  members  are  not  sharply 
distinct  and  the  contact  has  been  drawn  more  or  less  arbitrarily.  It  was 
intended  to  include  in  the  upper  member  all  the  pebble  beds  which  are  made 
up  of  rooks  similar  to  those  of  the  intercalated  flows. 

This  member  is  best  exposed  in  the  drainage  of  Windy  Gulch  and  on  the 
slopes  west  and  south  of  the  Commodore  Mine.  The  contacts  of  this  body  are 
fairly  well  mapped  except  where  it  is  in  contact  with  the  rocks  of  the 
upper  division  of  the  potosi  for  there  exposures  are  very  poor  and  the 
contacts  and  relations  are  uncertain.  The  flat  about  and  to  the  northwest 
of  Bachelor  are  included  in  this  member  although  much  of  this  may  be 
Quaternary  wash.  It  has  also  been  mapped  on  the  south  slope  of  Manmoth 
Mountain  but  this  area  is  almost  entirely  lacking  in  exposures  and  its  con- 
tacts are  very  greatly  generalized.  The  typical  rock  was  seen  only  in  a 
few  prospects  at  an  elevation  of  about  10,550  feet  just  west  of  the  isolated 

cliffs  of  Billow  Creek  rhyolite. 


/Of 


Petrography.-  3ie  clastic  rock  varies  greatly  in  tho  Degree  of  sort- 
ing and  rounding  of  the  fragments,  the  coarseness  of  the  material,  the 
petrographic  character  of  the  fragments,  and  the  amount  of  induration.  On 
the  whole  it  is  considerably  coarser  than  the  lower  member  of  the  Creede 
formation,  is  somewhat  more  indurated,  and  carries  abundant  fragments  of 
rock  similar  to  the  associated  flows.  A  small  part  of  the  material  is 
rather  thin  bedded  and  of  fine  shaly  texture,  somewhat  more  is  sandy,  while 
the  greater  part  is  conglomerate  or  breccia. 

A  large  part  of  the  material  is  rather  well  "bedded  and  sorted  and 
consists  of  fragments  up  to  several  inches  across,  seldom  well  rounded,  in 
a  large  amount  of  matrix.   The  fragments  are  chiefly  of  the  IJamnoth  Mountain 
rhyolite  with  some  of  the  "7illow  Creek  rhyolite,  but  locally,  west  of  Windy 
Quick,  they  are  almost  entirely  of  rocks  similar  to  the  associated  rhyolite 
flows,  especially  of  the  third,  banded  type,  of  massive  rock  (see  p.   ). 
Rare  specimens  of  rocks  resembling  the  ttidymite  latite  and  more  or  less 
similar  to  the  rhyolite  breccia  of  the  lower  division  of  the  potosi  are 
present;  other  rocks  are  rare.  The  matrix  is  dark  red  or  drab,  rarely 
white,  and  is  very  fine,  hard,  and  dense,  except  for  occasional  cavities, 
and  the  rock  superficially  resembles  a  flow  breccia.  The  cementing  material 
is  chiefly  quartz  and  chalcedony  with  considerable  iron.  Barite  and  jarosite 
were  found  in  a  number  of  places,  chiefly  in  the  cavities;  they  do  not 
appear  to  be  confined  to  the  vicinity  of  the  veins  as  they  are  common  in 
7indy  Gulch.  A  large  part  of  the  breccia  has  apparent ly  been  cemented 
by  ferfuginous  quartz  with  some  sulphates. 


The  fine  textured  layers  are  in  large  part  indurated  to  hard,  flinty 
rocks,  in  most  places  stained  red  or  drab  by  iron.  They  carry  a  few  poorly 
preserved  fragments  of  fossil  plants.  The  sandy  layers  in  the  main  consist 
either  of  broken  crystals  of  feldspar  and  biotite  or  of  fragments  of  fel- 
sitic  rhyolite  and  have  a  ferrugionous  quartz  cement. 

The  coarser  beds  of  conglomerate  which  are  interbedded  with  the  lava 
flows  in  .Vjndy  Oulch  and  near  the  Commodore  nine,  and  are  also  present  in 
lenses  in  the  overlying  breccia,  are  made  up  of  well  rounded  pebbles  up  to 
a  foot  or  more  across  with  a  moderate  amount  of  sandy  matrix.  They  are 
usually  cemented  by  silica  and  iron  compounds  to  a  hard  rock.  The  fragments 
are  nearly  all  of  rock  like  that  of  the  associated  flows;  in  some  places 
they  are  of  a  rock  with  more  prominent  phenocrysts.  Pebbles  of  the  latter 
type  are  always  much  altered. 

Massive  rocks.-  In  the  lower  part  of  the  section  in  the  drainage  of 
'tfindy  Gulch  and  to  the  northeast  as  far  as  the  Bachelor  Mine  a  number  of 
thin  flows  are  present  and  alternate  with  beds  of  conglomerate  made  up  of 
fragments  of  rocks  similar  to  that  of  the  flows.  They  have  not  been  separat* 
onthe  geologic  map  as  they  are  local,  thin,  poorly  exposed,  and  are  closely 
associated  with  the  conglomerates  which,  in  turn,  grade  into  the  normal  brec< 

The  greater  part  of  the  flows  is  made  up  of  a  rather  porous,  drab- 

I/ 

colored  rock  which  carries  phenocrysts  from  one  to  two  millimeters  in  cross- 
l/  Ridgway fs  quaker  drab  (1 f » •  • f )  to  purple  drab  ( 1» ' • f  -) . 

section,  of  glassy  orthoclase,  white,  porous  feldspar,  and  dark  brown  plates 
of  biotite.  These  phenocrysts  make  up  about  half  the  rock  and  are  imbedded 


in  a  porous,  aphanitic  inaterial.  The  microscope  reveals  a  few  quartz  grains, 
and  shows  that  the  white  feldspar  is  a  peculiar  microperthite  in  porous, 
skeleton  crystals  and  is  partly  altered  to  sericite.  This  gives  an  imper- 
fect wavy  extinction  as  the  intergrowthe  are  almost  submicroscopic.  Ortho- 
clase  predominates.  The  biotite  is  partly  resorbed.   Accessory  apatite, 
magnetite,  and  zircon  are  sparsely  present.  The  groundmass  shows  beautiful 
wavy,  fluidal  bands  of  lighter  tone  and  coarser  crystallization.   In  part 
the  crystallization  is  submicroscopic  but  irregular  streaks  and  bands  are 
made  up  of  closely  packed  spherulites,  here  minute,  there  large*  The  rocks 
are  fresh  except  for  the  presence  of  a  little  secondary  calcite  and  sericite. 
The  albite  may  have  been  derived  from  a  more  calcic  feldspar.  In  one  of  the 
sections  the  phenocrysts  have  very  ragged  borders  due  to  their  influence  in 
orienting  the  adjoining  groundnass.  The  groundmass  is  granophyric  and  rather 
coarsely  though  very  irregularly  crystalline.  Just  south  of  the  Comodore 
Mine  one  of  the  flows  shows  white  blotches  which  can  be  recognized  with  a 
pocket  lens  as  areas  of  radiating,  fibrous  spherulites. 

Another  type  of  massive  rock  found  only  west  and  south  of  the  Commodore 
Mine  carries  rather  numerous  rough  cavities  up  to  an  inch  in  largest  dimen- 
sion. It  has  fewer  phenocrysts  than  the  first  type  and  a  much  finer  ground- 
mass,  but  is  otherwise  similar. 

S0uth  of  Bulldog  Mountain  a  third  type  is  exposed  partly  as  a  massive 
rock  and  makes  up  much  of  the  breccias.  It  is  a  pale  purple-drab  rock  with 
rather  broad  bands  and  lenses  of  lighter  tone.  It  carries  rather  abundant 
crystals  of  glassy  orthoclase  and  a  few  of  white  feldspar  and  dark  brown 
biotite.  The  microscope  shows  that  the  plagioclase  is  near  albite  and  is 


somewhat  altered  to  sericite.  The  orthoclase  crystals  include  well  formed 
albite  crystals.   The  groundraass  of  the  broad,  dark  colored  bands  is  very 

finely  crystalline,  that  of  the  light  bands  is  rather  coarsely  crystalline 
with  the  quartz  and  orthoclase  intergrown.  The  fibers  or  prisms  of  feldspar 
tend  to  grow  out  from  the  walls  and  the  quartz  fills  in  between  them.  All 
the  flows  are  of  rhyolites  rather  rich  in  soda. 

feathering  and  Topography. 

The  beds  of  the  Creede  formation  are  by  far  the  least  resistant  of  any 
of  the  large  bodies  of  rock  within  the  Creede  Special  quadrangle,  or  in  its 
vicinity,  and  they  have  had  a  marked  influence  on  the  development  of  the 
topography.   The  broad  valley  of  the  Rio  Grande,  with  its  comparatively 
gentle  contiguous  slopes  from  Trout  Creek  to  Wagon  TCheel  Gap,  a  distance  of 
about  25  miles,  corresponds  to  the  area  occupied  by  these  tuffs  and  is  in 
marked  contrast  with  the  steep  rugged  slopes  surrounding  the  valley,  and 
with  the  canyon  of  the  Rio  Grande  both  above  and  below.   Equally  marked  is 
the  contrast  between  the  rugged  rock  canyon  of  Willow  Creek  above  Creede 
where  it  is  cut  in  the  billow  Creek  rhyolite  and  the  broad  valley  and  the 
rolling  hills  where  it  is  eroded  in  the  Creede  formation,  plate  jf. 'oppo- 
site page    shows  the  low,  rolling,  grass  covered  hills  of  tuff  about 
Creede  and  the  marked  change  in  topography  above  Creede  where  the  \7illow 
Creek  rhyolite  comes  out  from  under  the  tuffs.   This  is  also  shown  in 
plate  5F»  opposite  p. 


/ 


Origin. 

The  greater  part  of  the  clastic  material  of  the  Creede  formation  was 
clearly  deposited  by  water.  The  conglomerate  was  probably  deposited  in 
part  along  the  shore  of  the  lake,  in  part  by  contributory  streams  during 
freshets.  Near  Creede  conglomerate  is  much  more  abundant  in  the  upper 
part  and  makes  up  the  greater  part  of  the  clastic  material  in  the  upper 
member;  but  this  may  not  have  been  true  in  other  parts  of  the  basin.  In- 
deed the  upper  member  near  Creede  is  probably  largely  fluviatile  material. 
Ancient  talus  and  similar  subaerial  accumulations  make  up  a  small  part  of 
the  formation.   The  fine  shaly  tuff  must  have  been  deposited  in  compar- 
atively still  water  and  its  considerable  though  local  distribution,  to- 
gethir  with  its  great  thickness  and  the  form  of  the  basin  of  deposition 
are  rather  satisfactory  evidence  that  it  accumulated  in  a  local,  inter- 
volcanic  lake.   The  form  of  the  old  basin  beyond  the  present  Rio  Grande 
valley  is  not  known,  as  younger  rocks  hide  it  both  above  Trout  Creek  and 
beyond  ',7agon  'Sheel  (Jap. 

The  material  making  up  this  formation  was  derived  from  four  sources. 
Much  of  the  fine  tuff  represents  volcanic  ash  which  was  thrown  out  from 
some  unknown,  but  nearby,  volcanic  vent  and  fell  in  a  lake  or  on  sides  of 
the  basin  surrounding  it  and  was  deposited,  after  sorting,  in  a  quiet 
water  of  the  lake.   The  greater  part  of  the  coarse  material  was  derived 
by  the  ordinary  process  of  erosion  from  the  mountains  surrounding  the 
basin  and  was  brought  into  the  lake  by  the  torrential  streams  which  fed  it. 
This  portion  is  made  up  almost  entirely  of  the  underlying  rocks.  The 
travertine,  as  has  already  been  stated,  was  deposited  by  hot  springs 
contemporaneously  with  the  deposition  of  the  fine  tuff.   Finally,  a  number 

of  thin  lava  flows  are  present  in  the  upper  part  of  the  formation. 


Position  in  the  Section. 

The  Creede  formation  clearly  overlies  the  rocks  of  the  lower  division 
of  the  Potosi  as  the  contacts  and  relations  are  well  exposed  on  both  sides 
of  Creede  and  in  other  places  and  a  large  part  of  the  fragments  from  the 
coarse  material  in  the  Creede  formation  was  derived  from  the  two  rhyolites 
of  the  lower  division  of  the  Potosi*  it  also  overlies  the  rocks  of  the  upper 
division  of  the  Potosit  but  although  the  two  formations  are  in  contact  near 
Sunnyside,  in  the  drainage  of  ^7indy  Gulch,  and  near  Bachelor  the  exposures 
are  nowhere  sufficient  to  establish  the  relation  between  the  two.    The 
two  formations  are  also  in  contact  to  the  east  of  the  area  included  in  this 
report,  but  here  too  the  relations  are  obscure.  Although  the  Creede  for- 
mation everywhere  occupies  the  slope  below  the  upper  division  of  the 
Potosi,  the  form  of  the  contacts  indicates  that  the  Creede  was  deposited 
in  a  basin  with  steep  slopes  made  up  of  the  rocks  of  the  Potosi  volcanic 
series;  this  is  shown  more  clearly  to  the  east  of  the  area  included  in 
this  report  where  the  soft  tuffs  and  breccias  of  the  Creede  formation 
abut  against  steep  slopes  of  flows  of  the  Potosi  volcanic  series. 

On  the  whole  the  Creede  breccias  contain  fewer  fragments  of  rocks  a 
that  can  be  identified  as  belonging  to  the  upper  division  of  the  Potosi 
than  would  be  expected  from  the  relations,  but  just  east  of  Creede  frag- 
ments of  the  tridymite  latite  were  found  in  the  lower  member  of  the  Creede 
formation  and  in  7indy  Gulch  in  the  upper  member.  Moreover,  two  of  the 
formations  of  the  upper  division  of  the  Potosi  that  should  have  contributed 
largely  to  the  Creede  beds,  the  tfindy  Gulch  rhyolite  breccia  and  the 
Mammoth  Mountain  rhyolite,  weather  into  small  fragments  and  these  could 
hardly  be  distinguished  from  fragments  of  the  older  Campbell  Mountain 


rhyolite.  T0  the  east  in  the  drainage  of  Farmers  Creek,  where  the  Creede  beds 
lap  against  large  masses  of  rocks  belonging  to  the  upper  division  of  the  Potosi 
fragments  of  those  rocks  are  more  abundant  in  the  Creede  formation. 

In  the  area  included  in  this  report  none  of  the  rocks  have  been  found 
overlying  the  Creede  formation.  However,  at  Tagon  \Vheel  Sap,  only  eight 
miles  to  the  southeast,  a  thick  flow  of  a  hornblende-quartz  latite,  which 
is  without  doubt  related  to  the  Mac  Kenzie  Mountain  quartz  latite,  overlies 
the  tuff.  The  position  of  the  Creede  formation  is  9  therefore,  between  the 
MacKenzie  Mountain  quartz  latite  and  the  Potosi  volcanic  series. 

Age. 

The  thin  bedded  tuffs  of  the  Creede  formation  have  furnished  the  only 
determinable  fossils  found  in  this  vicinity.  These  tuffs  commonly  carry 
a  moderate  number  of  plant  remains,  some  of  which  are  well  preserved. 
Collections  were  made  from  three  localities  in  the  Creede  quadrangle  and 
submitted  to  P.  H.  Knowlton  for  identification. 

Following  are  the  lists  of  forms  reported  by  him  from  each  locality: 
2  (2951).  Creede  quadrangle,  Colorado.  Ridge  N.  of  Pierce  Creek,  near 
",7agon  'Theel  (Jap  Hot  Springs  Hotel,  at  9000  feet  elevation. 

Minute  fragments  of  bark,  coniferous  leaves,  etc.,  but  nothing  deter- 
minable. 

24  (5952).  South  bank  Rio  Grande  150  yds.  above  wagon  bridge  over  Rio  Grande, 

3|-  miles  below  Creede,  Colorado. 

Fontinalis  pristina?  Lesq. 

Cercocarpus  antiiuus?  Lesq.      A  very  narrow  leaf. 


93  (5953).  Tuff,  west  bank:  of  Rio  Grande  -J-  mile  north  of  Seven  Mile  bridge, 

seven  miles  above  Creede. 

Pinus  '.Theeleri  Cockerell.    Leaves  in  5*s. 

Sequoia  sp.  ? 

Thuya  sp. 

Dicotyledon,  narrow,  wedge-shaped  at  base;  no  nervation. 

^uercus  pyrifolia?  Lesq. 

Fontinalis  pristina  Lesq. 


_1.  This  has  been  shown  to  be  a  feather  and  not  a  moss. 

536  (6198).  Same  locality  as  93. 

Adiantites?  sp.  Itfew? 

planera  myricaefolia  (Lesq.)    Cockerell. 

Pinus  ,/heeleri  Cockerell. 

Pinus  sp.  cone,  like  P.  florissanti  but  smaller. 

Carduus  florissantensis  Cockerell. 

Vitis  florissantella  Cockerell. 

Conifer,  probably  Picea  or  Abies,  and  undoubtedly  new. 

LJahonia?  sp.  Trifoliolate  and  new. 

Abies,  two  species,  new. 

Juglans,  new? 

Celtis  mocoshii  Lesq. 

Lomatia  hakaefolia  Lesq. 

Ribis  protomelaeum  Cockerell. 

Insect,  beetle? 

Feather,  like  those  of  Florissant. 

Dr.  Khowlton  makes  the  following  comments  on  this  flora: 

"These  ppecies  are  all  or  nearly  all  found  in  and  are  highly  character- 
istic of  the  Florissant  flora.  The  question  then  arises  as  to  the  age  of 
the  Florissant  deposits.  For  many  years  they  were  supposed  to  be  in  the 
approximate  position  of  the  Green  River  formation,  but  on  the  determination 
of  the  fossil  insects  the  beds  have  of  late  years  been  referred  to  the  Oligo- 
cene.  Still  more  recently  extensive  explorations  have  been  made  in  the 
Florissant  plant  beds  with  the  result  of  bringing  to  light  a  great  number 
of  new  and  on  the  whole  of  very  modern  appearing  forms,  and  the  conviction 
has  been  growing  that  these  beds  had  been  placed  too  low  in  the  scale. 
Extensive  Tertiary  floras  from  this  country,  as  well  as  from  other  parts  of 
the  world,  have  lately  become  available  for  comparison,  and  it  now  seems 
probable  that  the  Florissant  beds  are  Upper  Miocene  in  age.  In  any  event 
I  feel  perfectly  justified  in  stating  that  the  plants  here  submitted  from 
the  vicinity  of  Creede,  Colorado,  are  of  the  same  age  as  the  plant  beds  of 
Florissant,  Colorado,  and  that  this  age  is  .certainly  Miocene,  and  in  all 
probability  upper  Miocene."   As  to  the  original  reference  of  these  beds  to 
the  Green  River,  I  can  only  say  that  none  of  the  plants  submitted  belong  to 
the  Green  River  flora,  while  all  but  one  a£  or  two  that  have  been  identified 
do  occur  in  the  Florissant  beds." 


Chapter  6. 

Latest  series  of  lava  flows. 

General  statement. 

In  the  Creede  and  Vicinity  quadrangle  a  still  later  volcanic  series 
is  represented  about  Mackenzie  Mountain  by  a  great   lava  flow  here  named 
the  Mac^nzie  Mountain  quartz   latite.       Beyond  the  quadrangle  in  nearly 
all  directions,   this  latest  volcanic  series  is  extensively  developed  and 
attains  a  great   thickness  on  the  divides  both  to  the  north  and  south  of 
the  Rio  Grande  and  tongues  and  patches  of  it   extend  toward  the  Rio  Grande 
Valley  and  locally  cross  the  valley.     It   is  made  up  of  a  succession  of 
lava  flows  and  pyroclastic  deposits.       some  of  the  flows  are  several 
hundred  feet   thick  and  persits   for  long  distances;   others  are  irregular 
and  local.     The  pyroclastic  part  is  largely  very  chaotic  and  contains 
rather  abundant   irregular  bodies  of  flow  rock.     The  rocks  of  this  series 
are  quartz  latites  and  rhyolites  and  are  nearly  all  characterized  by 
larger  and  more  conspicuous  phenocrysts  than  are  the  other  rock  of  the 
area  and  they  can  usually  be  recognized  with  little  difficulty. 

The  relation  of  this  volcanic  Series  to  the  potosi  volcanic  series 
is  well  shown  in  the  vicinity  of  MacKenzie  Mountain  where  the  great   lava 

flow,  here  named  Mackenzie  Mountain  quartz  latite,   overlies  the  various 

& 

members  of  the  Potosi  volcanic  series  in  an  irregular  manner  and  clearly 

flowed  over  an  erosion  surface  of  considerable  relief  and  filled  in  the 
valleys  and  other  irregularities.   Beyond  the  limits  of  this  quadrangle 


//o 


in  Upper  Bast  billow  Creek,  and  elsewhere,  the  irregularity  at  the  base  of 
this  series  is  still  more  marked  and  is  persistent  wherever  this  series  has 
been  found. 

The  relation  of  this  volcanic  series  to  the  Creede  format ionis  not 
shovm  on  the  area  covered  by  this  report  as  the  two  are  nowhere  in  contact. 
However,  about  8  miles  to  the  east,  at  Wagon  Thoel  Gap,  a  great  flow  be- 
longing to  this  volcanic  series  rests  successively  on  the  Greede  formation 
and  rocks  of  the  Potosi  volcanic  series  and  has  a  lower  contact  which  cuts 
across  the  contours  very  irregularly.   The  hard  rock  which  forms  the  Gap 
is  of  this  formation.  There  was  clearly  a  considerable  interval  of  erosion 
succeeding  the  deposition  of  the  Creede  lake  beds  and  preceding  the  extrava- 
sation of  the  rocks  of  this  upper  volcanic  series. 

MacKenzie  Mountain  quartz  latite. 
General  character  and  distribution. 

On  Mac^nzie  Mountain  and  the  ridge  to  the  north  and  south  is  a  prominent 
development   of  a  quartz  latite  characterized  by  abundant  green  augite  crystals 
and  large  and  prominent  phenocrysts  of  feldspar  and  biotite.     Within  the  area 
mapped  it  probably  represents  a  single  flow,  although  a  part  may  represent 
material  which  solidified  within  the  vent  through  which  the  material  of 
the  flow  was  extruded.       As  shown  on  Plate  II,  this  rock  occupied  the  large 
area  including  Mackenzie  Mountain  and  two  smaller  areas  on  the  same  ridge 
to  the  south  of  the  Xreutzer  fault.     In  addition,   there  is  a  small  outcrop 
of  this  rock  surrounded  by  glavial  drift   in  upper  Rat  Creek,  another  just 
east   of  Hat  Creek,   and  a  third  west  of  Rat  Creek  and  north  of  east   of 
MacKenzie  Mountain.     This  latter  body,  which  caps  the  hill  with  10,800  contour 


is  poorly  exposed  and  may  represent  slide  rock  from  the  cliffs  above,  although 
it  is  believed  to  be  in  place.  Similar  rocks,  which  no  doubt  represent  the 
same  eruptive  series  although  not  the  same  flow,  are  extensively  developed 
on  the  higher  slopes  both  to  the  north  and  south  of  the  Rio  Grande, 

Character  and  thickness. 

'Vherever  observed  this  flow  carae  o'ver  a  surface  of  relief  comparable 
with  that  of  the  present  surface  and  within  the  Greede  Special  quadrangle 
the  extreme  irregularity  of  its  base  is  well  shown.  On  the  east  side  of  the 
saddle  which  is  just  north  of  MacKenzie  Mountain  the  base  of  this  flow  is 
well  exposed  at  an  elevation  of  11,150  feet;  from  here  to  the  southeast  the 
base  of  the  cliff,  which  is  practically  at  the  base  of  the  flow,  as  is  in- 
dicated by  the  glassy  rock  at  this  horizon,  cuts  sharply  across  the  contour 
and  within  a  distance  of  an  eighth  of  a  mile  crosses  the  10,850  contour, 
a  fall  of  300  feet.   Just  north  of  this  the  steep  slope  of  the  base  is 
even  more  striking  as  it  is  also  on  all  sides  of  the  isolated  body  on  the 
west  side  of  Rat  Greek.  This  latter  body  evidently  rests  on  very  steep 
slopes  of  the  older  rocks  and  it  was  at  first  believed  to  be  intrusive 
but  the  identity  of  the  rock  with  that  of  the  flow  across  the  creek,  and 
the  glassy  horizon  at  its  base,  together  with  the  details  of  the  contacts, 
/here  exposed,  show  that  the  body  is  a  flow  resting  on  the  rocks  of  the 
upper  division  of  the  Potosi  series. 

Owing  to  the  extreme  irregularity  at  the  base  of  this  flow  its  thickness 
is  variable.  Moreover,  the  top  is  nowhere  certainly  exposed,  although  the 
small  plateau  to  the  northwest  of  MacKenzie  Mountain  is  believed  to  repre- 
se  t  nearly  the  top  of  this  flow.  It  is  commonly  several  hundred  feet 
thick  and  locally  as  much  as  500  feet. 


Petrography. 

Megascopic.-  The  rock  is  fairly  uniform  in  appearance,  and  varies  chief- 
ly in  color,  porosity  of  the  groundraass  and  the  amount  of  glass  in  the  grounc 
mass.  The  color  is  purple-drab  (l»flf)  in  thfee  denser  rock  to  light  quaker 

I/  i/ 

drab   (lf?fffb)     or  light   mouse  gray  (15ttf"b)      in  the  more  porous  rocks. 

"    I/ 
The  glassy  horizon  at   the  base   is  deep  olive-gray  (23 »        r  -). 

I/    Ridgway's  names  and  symbols. 

MW_M__M<*_  ^m*m»  ^~+  — *  MPOV  ^^»  -•  ^  ^••^••^•w  «—  —  m»«»^  «•  •—  ^•WM—  ^^•^-•••^^^^  «••*••  — •  —*^mm*~  —•  ^  ^»  «• 

Except  for  a  glassy  layer  at  the  base,  all  the  rocks  show  a  few  small  gas 
pores  and  some  of  the  rocks  are  decidedly  porous.  The  flow  is  characterized 
by  the  mmber  and  especially  the  size  of  its  phenocrysts  which  about  equal 
the  groundmass  in  amount  and  are  commonly  from  2  to  4  millimeters  across, 
occasionally  a  centimeter  or  more.  They  are  chiefly  of  porcelain  white 
plagioclase  with  rather  prominent  black  flakes  of  biotite  and  prisms  of  pale 
green  augite.  The  groundmass  is  aphanitic. 

Microscopic*-  In  addition  to  the  minerals  recognized  with  a  pocket 
lene  the  microscope  shows  a  few  crystals  of  zircon,  apatite,  and  magnetite. 
The  groundmass  is  largely  submicroscopic  in  crystallization  and  is  rhyo- 
litic  in  character.  The  plagioclase  is  andesine-labradorite  in  composition 
and  carries  central  inclusions  of  the  other  constituents.  The  biotite  is 
considerably  resorbed  and  the  augite  somewhat  alteredtto  chloritic  material. 


7/eathering  and  outcrops* 

roclc  is  little  altered;  it  is  very  hard  and  resistant  to 
weathering  and  nearly  everywhere  forms  prominent  cliffs.   It  is  under- 
lain by  softer  rocks  and  landslides  have  been  important  factors  in 
breaking  up  the  flow.  The  slopes  beneath  its  cliffs  are  commonly 
covered  with  a  heavy  mantle  of  landslide  material  from  this  cliff- 
forming  quartz  latite. 


Chapter  VII. 
Intrusive  rocks. 
General  statement. 

In  the  Greede  and  Vicinity  quadrangle,  intrusive  rocks  are  neither 
great  in  extent,  numerous  in  individual  bodies,  nor  varied  in  character. 
Three  types  have  been  recognized,  rhyolite  porphyry,  quartz  latite  por- 
phyry* and  basalt.  The  rhyolite  porphyry  is  fairly  uniform  in  character 
and  was  probably  all  intruded  at  about  one  time,  during  the  latter  part 
of  the  period  represented  by  the  lower  division  of  the  Potosi.  It  is 
found  south  of  Bulldog  Mountain  and  east  of  billow  Creek  in  irregular 
or  sill-like  intrusions,  none  of  which  are  of  large  extent.  The  quartz 
latite  porphyry  is  very  uniform  in  character  and  no  doubt  the  intrusions 
were  formed  at  about  the  same  time.   It  is  confined  to  the  southwest 
corner  of  the  quadrangle  and  occurs  in  dikes  up  to  a  few  hundred  feet 
across,  striking  west  of  north.   It  is  probably  of  about  the  same  age 
as  the  upper  quartz  latite  and  may  be  the  intrusive  equivalent  of  that 
or  a  related  flow.  The  basalt  was  found  in  only  one  narrow  dike  in 
Lower  Rat  Creek,  and  cuts  the  lower  part  of  the  upper  division  of  the 
Potosi. 


Rhyolite  porphyry. 
General  character  and  distribution. 

A  nearly  white  rhyolite  porphyry,  characterized  by  large,  glassy 
crystals  of  orthoclase  is  present  in  several  considerable  sized  bodies 
to  the  south  of  Bulldog  Mountain  and  in  some  smaller  bodies  on  the 
slopes  west  of  7/illow  Creek  just  above  Creede.   Small  bodies  of  this 
rock  were  found  in  some  of  the  underground  workings  and  a  body  of 
somewhat  similar  rock  is  exposed  on  both  sides  of  West  pillow  Creek 
just  below  '.yeaver.   The  details  of  the  distribution  are  shown  on 
Plate  II. 

This  rhyolite  porphyry  is  very  soft  and  gives  few  good  outcrops. 
T0  the  south  of  Bulldog  Mountain,  in  particular,  outcrops  are  almost 
entirely  wanting  and  the  boundaries  were  mapped  largely  from  the  soil 
and  talus.  The  character  of  contacts  was  rarely  observed  and  the 
mapping  is  therefore  generalized. 

Relation  to  adjoining  rocks. 

Wherever  exposures  are  sufficient  to  offer  any  data  as  to  the  re- 
lation of  this  rhyolite  porphyry  to  the  adjoining  rocks  an  intrusive 
origin  is  strongly  suggested.   It  nowhere  appears  to  occupy  a  definite 
horizon  in  the  section,  although  it  is  confined  to  the  rocks  of  the 
lower  division  of  the  Potosi  and  it  is  commonly  between  the  two  rhyo- 
lites  of  that  division,  or  near  their  contact.  The  two  small  mapped 
bodies  on  the  east  side  of  the  ridge  south  of  Bulldog  Mountain  are 


//L 


clearly  cross  cutting  as  shown  by  their  contacts  where  exposed  in  several 
places.  The  other  bodies  on  this  ridge  are  so  poorly  exposed  that  their 
relations  could  not  be  determined.   They  probably  represent  in  part 
gently  dipping  dikes  or  sills,  perhaps  in  part  irregular  intrusives.  The 
small  masses  of  this  rock  just  north  of  Creede  are  somewhat  better  exposed 
and  are  in  part,  at  least,  crosscut  ting  bodies.  Dikelike  bodies  of  this 
rock  are  crossed  by  the  Kelson  Tunnel.  The  body  south  of  Weaver  has  the 
form  of  a  sill* 

The  porous  character  of  the  rock  suggests  a  surface  rock,  while  the 
texture  rather  suggests  an  intrusive.  The  intrusion  probably  took  place 
very  near  the  surface  and,  indeed,  a  considerable  part  of  the  rock  may  have 
reached  the  surface. 

Petrography. 

Megascopic.-  In  luster  the  rocks  are  dull  and  chalky,  in  color  they 
are  nearly  pure  white  with  a  very  slight  vinaceous  cast  and  a  few  grade  to 
quaker  drab.   They  are  commonly  somewhat  porous  and  some  carry  abundant 
very  fine  pores.  Much  of  the  rock  shows  an  imperfect  fluidal  banding. 
It  carries  a  few  thick  tablets  of  glassy  orthoclase  nearly  a  centimeter 
across  in  a  very  fine-textured  to  aphanitio  groundmass.  Dark  minerals 
are  almost  entirely  wanting.  The  rock  just  wouth  of  leaver  is  much 
altered  and  differersoaewhat  from  the  -est  of  the  rock.  In  addition  to 
the  usual  large  orthoclase  it  carries  a  few  crystals  of  quartz,  biotite, 
and  of  a  completely  altered  feldspar,  probably  a  plagioclase.  It  has 
a  somewhat  brecciated  appearance. 


Micro scopic*-  The  tnin  sections  show  that,  in  addition  to  the  large 
phenocrysts  of  orthoclase,  there  are  a  very  few  partially  resorbed  crystals 
of  biotite  and  the  usual  accessories  -  apatite,  zircon,  and  magnetite. 
Quartz  phenocrysts  and  kaolinized  feldspar,  probably  plagioclase,  were  found 
only  in  the  specimens  from  the  body  south  of  ',7eaver.  The  groundmass  is 
irregular  in  texture.  It  carries  very  abundant  grains  of  quartz  up  to  a 
few  tenths  of  a  millimeter  in  cross  section,  and  a  few  of  orthoclase; 
these  are  imbedded  in  and  are  not  sharply  distinct  from  a  very  fine  matrix 
of  quartz  and  orthoclase.  The  quartz  of  the  groundmass  ad.jace.nt  to  the 
quartz  grains  has  oriented  itself  with  respect  to  these  grains  and  is 
crystallographically  continuous  with  then. 

Quartz  latite  porphyry. 
Occurrence. 

In  the  drainage  of  Miners  Greek  and  to  the  west  there  are  a  number 
of  dikes,  some  of  them  several  hundred  feet  across,  of  a  quartz  latite 
porphyry  of  rather  uniform  character.  Most  of  these  dikes  are  nearly 
vertical  and  strike  a  little  west  of  north.  To  the  west  of  the  boundary 
of  the  quadrangle  are  several  more  dikes  of  the  same  character,  and 
striking  in  about  the  same  direction  with  some  tendency  to  radiate  from 
the  hill  north  of  MacKenzie  Mountain. 

The  difee  on  the  west  boundary  of  the  quadrangle  is  the  largest  of 
those  shown,  although  some  to  the  west  are  still  larger.  This  dike  is 
over  200  feet  wide  in  places  and  has  been  followed  for  over  a  mile.  The 
dike  Just  west  of  Miners  Greek  is  somewhat  narrower  and  is  about  three- 
quarters  of  a  mile  long.  The  bodies  near  the  fault  to  the  east  of  '.liners 


Creek  probably  represent  three  distinct  dikes,  two  of  them  about  half  a  mile 
and  the  other  only  a  few  hundred  feet  in  length.  The  southern  of  these  dikes 
was  broken  by  the  fault.  Four  small,  poorly  exposed  bodies  are  mapped  in  the 
southwest  part  of  Seo.  34,  and  poor  exposures  of  another  small  body  were 
found  on  both  sides  of  Rat  Creek  in  the  southern  part  of  Sec.  27.  The  body 
on  the  quadrangle  line  west  of  Mac^nzie  Mountain  is  a  part  of  a  larger  body 
most  of  which  is  beyond  the  limits  of  the  quadrangle.  The  dikes  are  rather 
resistant  to  weathering  and  commonly  give  prominent  outcrops.  3/here  they 
cut  soft  rocks  they  stand  out  as  broken  walls. 

Age* 

These  dikes  clearly  cut  the  rhyolites  of  the  lower  division  of  the 
potosi  and  to  the  west  they  cut  the  rocks  of  the  upper  division  of  the 
Potosi  reaching  as  high  in  the  section  as  the  andesite.  They  are  older 
than  the  last  movement  along  the  Xreutzer  fault  but  may  possibly  be  younger 
than  the  main  movement,  as  will  be  shown  in  the  chapter  on  structure.  This 
rock  resembles  the  upper  quartz  latite  of  Mackenzie  Mountain  both  in  com- 
position and  habit,  and  in  places  to  the  north  of  Mackenzie  Mountain  there 
is  some  indication  that  the  two  grade  into  each  other.  They  are,  therefore, 
of  about  the  same  age  as  the  upper  quartz  latite  and  probably  represent  the 
channels  through  which  this  or  very  similar  and  closely  related  flows  were 
extruded. 


Petrography. 
Megascopic*-  In  color  the  rocks  vary  somewhat  but   are  with  few  except io 

IT 

gray.  They  are  dense  and  show  phenocrysts,  commonly  several  centimeters 

I/  Near  Ridgway's  light   olive  gray  (23»»f*M)   or  light  mouse  gray  (15fft»»  d 

•»••  m**mm*  ••»»  —  •».  w  *•»  ^  4M  •*  •••«•»  •»•»••  •»  ••  «••  w  ••>  ^*MM»  «»•••»  ^M**M.B^«»  v  v  ••  ^  w  «»  •»  «»••.«  «  ^  «w.  ~' v  ••*  •— M.  _  «.  v  •_  ••  ».  „  ^  «v  ^«*  ^«w«*  ^  ••—.•••• 

across,  nearly  equal  in  amount  to  the  aphanitic  groundmass.  The  phenocrysts 
are  chiefly  of  white  plagioclase  in  thick  plates  with  rounded  to  hexagonal 
outline.  Biotite  is  rather  abundant  and  in  some  of  the  material  augite 
and  less  commonly  hornblende  are  present.  Phenocrysts  of  orthoclase  and 
quartz  are  rare. 

Microscopic.-  The  microscopic  study  shows  that  the  plagioclase  pheno- 
crysts are  not  greatly  zoned  and  are  andesine-labradorite  in  average  compo- 
sition. The  biotite  is  commonly  resorbed.  The  accessories  are  apatite, 
zircon,  and  magnetite.  Less  than  half  of  the  rocks  show  remnants  of  augite, 
largely  altered  to  calcite  and  chlorite;  nearly  all  show  areas  of  calcite 
and  chlorite  which  probably  represent  altered  augite;  a  few  show  in  addition 
a  little  green  hornblende.  The  groundmass  is  usually  a  rather  coarse  micro- 
graphic  intergrowth  of  quartz  and  orthoclase  with  some  small  laths  of 
plagioclase.  The  rocks  are  comnonly  somewhat  altered  with  the  development 
of  calcite  and  sericite  from  the  plagioclase  and  of  oalcite  and  chlorite 
from  the  augite. 


Intrusive  basalt. 

Occurrence. 

A  single  small  dike  of  dark  colored  rock  was  found  on  the  quadrangle. 
It  cuts  the  rocks  of  the  upper  division  of  the  Potosi  as  high  in  the  section 
as  the  V7indy  Grulch  rhyolite  breccia  on  the  slopes  west  of  Rat  Greek  in  the 
southern  part  of  Sec.  22.   It  has  a  width  of  about  10  feet  and  strikes 
nearly  north  and  south.  It  is  rather  poorly  exposed. 

Petrography. 

!/ 

The  fresh  rock  is  iron  gray     ,  but  on  exposure  it   bleaches  to  deep 

I/     Hidgway's    (23    •••••  K). 

_ _  ______ ——•.————— «.•••————•••— —..———-•— —••——— ——«••.———•«—————-•—-•———•———•••———————————— 

£/ 

olive  gray.    The  rock  is  dense  and  shows  to  the  naked  eye  a  very  few 

_  ___________-.—— — .— — _  ————————————— «•— — — — — —  —————————  — — .. — — — — — — — < 

2/  Ridgwayf8  deep  olive-gray  (23  »»»•«  -). 

__—__..——————.-—————-—————————————-—-———— ————..—.————————— — — — — — — — ----- 

glassy  crystals  of  plagioclase.   With  a  pocket  lens  numerous  minute  laths 
of  plagioclase  can  be  recognized  by  their  bright  cleavage  faces. 

Thin  sections  of  the  rock  show  that  it  is  made  up  in  large  part  of 
small,  thin  laths  of  plagioclase,  arranged  nearly  parallel  from  flow  and 
imbedded  in  a  smaller  amount  of  glass  clouded  with  opacite  and  with  in- 
cipient crystallization.   Augite  grains  and  prisms  are  rather  abundant 
and  olivine  was  originally  about  equally  abundant  but  is  now  altered  to  a 
rather  strongly  birefracting  fibrous  serpentine.   The  plagioclase  crystals 
are  labradorite.   The  usual  apatite  and  black  iron  ore  are  present.  The 
rock  is  fairly  fresh  except  for  the  Berpentinization  of  the  olivine  and 
the  deposition  of  a  little  secondary  calcite. 


Chapter  VIII. 

Quaternary  deposits. 
Int  r oduct  ory  s  tat  ement  • 

A.  comprehe naive  Imowledge  of  the  topography  and  of  the  Quaternary  deposits 
is  important  to  the  geologist  as  it  enables  him  to  interpret  more  or  less  im- 
perfectly the  geologically  recent  history  of  the  area,  such  as  the  extent  and 
character  of  the  glaciation  and  the  carving  out  of  the  present  mountains  and 
valleys.  Such  a  Imowledge  should  be  of  considerable  value  to  the  miners  and 
prospectors  as  it  aids  in  an  understanding  of  enrichment,  of  the  disappearance 
of  some  veins  along  the  strike,  such  as  the  Amethyst  vein  to  the  north,  and 
of  other  important  features  of  the  ore  deposits.  In  the  early  days  of  the 
Creede  camp  it  would  have  enabled  the  prospectors  and  operators  to  avoid  the 
useless  driving  of  tunnels  and  shafts  into  terminal  moraines  or  great  land- 
slides. 

As  may  be  seen  on  the  geologic  map  (Plate  II),  of  Creede  and  vicinity, 
Quaternary  deposits  cover  considerable  areas  in  this  district;  on  the  basis 
of  their  origin  and  relative  age  they  have  been  divided  on  the  map  into  a 
number  of  units*. 

The  earliest  of  these  deposits  with  the  possible  exception  of  some 
gravel  deposits  near  Bachelor,  are  represented  by  the  terminal  moraines  of 
the  earlier  glaciers  and  their  outwash  deposits  which  are  probably  repre- 
sented by  the  upper  terrace  deposits.  Following  this  were  formed  the  moraines 
of  the  later  glaciers  and  their  outwash  benches.  Nearly  or  quite  all  of  the 
landslide*  were  developed  later  than  the  glaciation.  The  alluvium  fans  and 
alluvium  are  in  large  part  recent  deposits  and  have  been  enlarged  and 
modified  up  to  the  present  time. 


Moraines. 

Distribution. 

Thousands  of  years  ago,  but  geologically  only  one  short  period  ago, 
the  high  mountains  and  divide  to  the  north  of  Creede  were  covered  by  a  great 
thickness  of  perennial  snow.  These  great  snow  fields  were  the  gathering 
grounds  for  many  glaciers  which  occupied  the  upper  valleys  of  all  the 
larger  streams,  such  as  Miners  Creek,  Rat  Greek,  ani  both  forks  of  Willow 
Creek.  The  glaciers  in  these  streams  did  not  reach  the  valley  of  the  Rio 
Grande  but  ended  at  an  elevation  of  about  ten  thousand  feet.  The  glacier 
that  occupied  the  upper  basin  of  Rat  Creek  and  those  of  the  two  forks  of 
Willow  Creek  came  into  the  area  included  in  the  Creede  and  Vicinity  map 
but  that  of  Miners  Creek  ended  a  mile  or  so  northwest  of  that  area. 

Glacial  topography. 

In  going  up  any  of  the  main  streams,  such  as  T7est  Willow  Creek,  one 
is  struck  by  the  marked  change  in  the  character  of  the  valleys  in  passing 
from  the  lower  unglaciated  to  the  upper  glaciated  part.  Below  the  glacia- 
tion  the  valleys  are  all  strikingly  V-shaped  canyons  with  steep  rock  walls 
coming  to  a  sharp  angle  in  the  creek  bed.  In  the  glaciated  part  the  val- 
leys open  up  and  have  almost  continuous,  nearly  flat  parks  and  meadows 
near  the  streams.  They  are  U-shape  in  cross  section  in  contrast  to  the 
V- shape  of  the  lower  unglaciated  parts.  »gfcirg---»e«*ske*a^»'- i>l^%^B^ya^aM-Vb^- 

Two  stages  of  glaciation. 

I/ 
Three  stages  of  glaciation  in  the  Quaternary  have  been  recognizedT^ 

•>  *~flB  —  M»  *w^  -v  «•  w  —  ••  *_  *•  ^  av«v  w»  ^««*»  •»•»  —  •—-»!•••  ^  ^  ••  •"  •"  wmmm+m^  ^^mm**mm^  •  ^  ^^m^mmmm  ^  «»^*«  • 

I/  Atwood,  ,7.  7.,  and  Mather,  K.  P.,  The  evidence  of  three  distinct  epochs  in 
the  pleistocene  history  of  the  San  Juan  Mountains,  Colorado,  Jour.  Geol.,  vol, 
20,  pp.  385-409,  1912. 


in  the  San  Jtian  Mountains.     Each  of  these   stages  covered  a  long  period  of 
years  and   in  each  the  glaciers  crept  down  the   vail  ley  and  then  gradually 
receded.       After  a  long  interval  the  glaciers  again  increased  in  size  and 
the  next   stage  began.     Only  two  of  these  stages  have  been  recognized  near 
Creede  and   their  deposits  are  not   easily  distinguished  from  each  other. 

Earlier  Moraine. 

The  earlier  glacier  of  Rat  Creek  probably  ended  northeast   of  MacXenzie 
Mountain  and  the  gently  sloping,  hummocky  area  just  north  of  the  hill  with 
the  10,800-foot  contour  represents  its  terminal  moraine.     The  numerous  large 
landslides  of  this  area  have  masked  the  glacial  material  to  a  considerable 
extent.      In  'Test    Billow  Creek  the  earlier  glacier  reached  nearly  to  the 
mouth  of  Nelson  Creek  and  may  have  crossed  the  ridge  into  Nelson  Creek. 
Its   terminal  moraine  has  the  usual  hummocky,    imperfectly  drained  surface 
but    it   is  not   so  well  preserved  as  is  that   of  the  later  glacier.         The 
separation  of  the  two  moraines  is   only  approximate.        The  earlier  glacier 
reached  nearly  or  quite  to  Phoenix  Park  in  East  Willow  Greet  and  its 
morainal  material  extends  for  only  a  short  distance  up  the  creek  before 
being  covered  fcy  the  later  moraine. 

Later  moraine. 

In  the  Creede  area  the  later  glaciers  reached  to  within  a  mile  or  so 
of  the  maximum  extension  of  the  earlier  glaciers.       Their  terminal  moraines 
are  somewhat  better  preserved  than  are  those   of  the  earlier  stage  and  in 
general  the  surfaces  of  their  moraines  are  but  little  modified  and  have 
the  hummocky  surface,   the  lack  of  drainage,  the  numerous  ponds  and  lakes 


characteristic  of  glacial  moraines.   In  rat  Creek  much  of  the  area  of  the 
moraine  is  on  steep  hillsides  but  in  both  forks  of  Willow  Creek  considerable 
areas  have  little  relief* 

Character  of  material. 

Gravels  make  up  nearly  all  of  the  morainal  deposits.  The  pebbles  are 
of  the  volcanic  rock  from  the  upper  part  of  the  drainage  basins,  are  well 
worn,  and  are  in  large  part  rather  fresh.  Many  are  flat  and  soraa  show 
typical  glacial  striation  although  on  the  whole  they  are  not  of  such  a 
character  as  to  preserve  the  markings  well. 

Post  Glacial  Erosion. 

S^nce  the  disappearance  of  the  glacial  ice  the  agents  of  erosion  have 
been  active  but  have  as  yet  modified  the  canyons  but  little.  In  East  pillow 
Creek,  about  half  a  mile  southeast  of  the  northeast  corner  of  the  area  in- 
cluded in  the  map  the  creek  is  in  a  canyon  about  100  feet  deep  with  a  nearly 
vertical  wall  of  quartz  latite  on  the  east  and  steep  slopes  in  the  glacial 
debris  on  the  west.   This  is  exceptionally  deep  for  the  post  glacial  can- 
yon and  is  very  local  and  cut  in  comparatively  soft  rocks.  In  most  places 
the  post-glacial  erosion  has  been  slight. 

EC onoraic  cons iderat  ions . 

Much  prospecting  has  been  done  in  the  glacial  material  especially  in 
East  7illow  Creek.  Pew  of  the  prospects  have  reached  bedrock  as  the  glacial 
cover  has  a  considerable  thickness  even  when  deposited  against  steep  hill- 
sides. Anyone  prospecting  in  t.iis  material  should  understand  that  the 


mineral  veins  do  not  extend  into  the  glacial  material,  that  the  glacial 
debris  varies  greatly  in  thickness  and  it  is  in  most  places  more  than  a 
thin  mantle;  and  that  it  was  deposited  in  a  valley  comparable  in  form  to 
the  present  valley  but  somewhat  deeper.   Tunnels,  especially  in  the 
flatter  parts  of  the  surface  will  generally  have  to  be  run  for  considerable 
distances  before  bedrock  is  reached. 

Terrace  gravels. 

In  the  valley  of  the  Rio  Grande  to  the  south  and  in  the  lower  parts  of 
the  smaller  valleys  are  two  rather  prominent  gravel-covered  terraces  respec- 
tively about  50  and  100  feet  above  the  streams.     These  terraces  are  remnants 
of  older  valleys,   somewhat  higher  than  the  present   valleys  and  considerably 
broader.        The  upper  terrace  was  probably  formed  during  the  earlier  glacial 
stage  by  the  streams  ^hich  emerged  from  the   lower  ends  of  the  glaciers  and 
resulted  from  the  melting  of  the  ice.     The   lower  terrace  was  probably  formed 
during  the  later  glacial  stage.        In  addition  to  the  two  rather  prominent 
and  nearly  continuous  terraces  are  remnants  of  other  higher,  gravel-covered 

terraces. 

These  terrace  gravels  were  found  only  in  the  lower  parts  of  the  streams 
and  are  in  large  part  cut  in  the  softer  rocks.  They  extend  up  the  valley 
of  Rat  Creek  for  some  distance.  The  details  of  their  distribution  are 
shown  on  Plate  II.  As  topographic  features  they  are  rather  prominent  especiall; 
in  the  valley  of  the  Rio  Grande  just  south  of  the  area  included  in  the  map. 
They  form  rather  gently  dipping  benches  on  both  sides  of  the  river  and  have 
been  dissected  by  the  larger  gullies.  The  benches  are  covered  with  gravels 
made  up  of  well-rounded  bowlders  with  considerable  finer  material,  and 
probably  represent  material  carried  down  from  the  glaciers. 


Landslides* 
**eneral  statement. 

In  the  Creede  area,  as  in  other  parts  of  the  San  Juan  Mountains,  land- 
slides are  abundant  and  some  are  of  considerable  size.   Favorable  condi- 
tions for  their  development  are  a  steep  slope  cut  in  rather  thick  lava 
flows  which  overlie  soft  tuffs;  thick  flows  such  as  those  of  the  lower 
rhyolites  are  not  favorable.   In  general  the  slopes  of  the  canyons  of  the 
four  main  streams  are  very  steep  and  the  glaciated  parts  are  especially 
favorable  for  landslides. 

Landslides  were  no  doubt  formed  before  the  glaciation  of  the  region, 
but  the  greater  part  at  least  of  those  shown  on  the  map  are  post-glacial, 
and  their  formation  has  continued  to  the  present  time. 

Description. 

In  the  mapping,  those  bodies  have  been  considered  landslides  which 
v/ere  formed  by  large  masses  of  rock  moving  down  the  slopes  more  or  less 
as  units.   In  some  cases  a  great  mass  of  rock  broke  from  the  top  of  a 
mountain  and  moved  rapidly  down  the  slopes;  in  many  the  movement  took 
place  in  stages.  Some  of  the  areas  mapped  as  landslide  represent  aggre- 
gates of  a  number  of  small  slides.  Near  the  mouth  of  the  Nelson  Tunnel 
the  material  is  merely  talus.  In  their  surface  forms  landslides,  like 
glacial  moraines,  are  characterized  by  hummocks,  lakelets,  and  imperfect 
drainage.  However,  the  sags  and  ridges  of  landslides  are  lateral  to  the 
valleys,  while  those  of  moraines  are  transverse;  moreover,  landslides 
commonly  show  jagged  cliffs,  pinnacles,  and  similar  forms. 


The  material  making  up  a  landslide  is  derived  from  the  slopes  above,  and 
commonly  from  a  few  or  even  a  single  rock  body  -  the  harder  rock  from  near 
the  crest  of  the  mountain.  The  fragments  are  characteristically  angular 
and  some  are  hundreds  of  feet  across.  There  is  no  sorting  and  the  arrange- 
ment is  prominently  chaotic.  3  me  of  the  great  blocks  resemble  cliff  out- 
crops of  rock  in  place  except  for  their  inconsistent  structures  and  rela- 
tions and  general  "jumbled  up"  arrangement. 

Distribution. 

The  distribution  of  landslides  is  shown  on  Plate  II  and  only  a  few 
special  features  will  be  discussed  here.   No  attempt  has  been  made  to  map 
any  but  the  larger  bodies.  The  large  slide  northwest  of  Bulldog  Mountain 
was  formed  by  the  slumping  of  great  blocks  of  the  thick,  resistant  flow 
of  tridymite  latite,  which  is  here  underlain  by  soft  beds  of  tuff.   It  is 
made  up  of  exceptionally  large  blocks,  some  of  which  give  nearly  vertical 
cliffs  of  the  latite  but  little  broken,  and  as  much  as  a  hundred  feet  in 
height.  However,  the  chaotic  arrangemant  and  the  undrained,  hupnocky 
topography  are  characteristically  those  of  a  landslide  area. 

The  landslide  southwest  of  Nelson  Mountain  is  made  up  in  large  part  of 
the  ii n^iuiu  ntniiLl4J.it  JOTl  quartz  latite  tuff  and  has  some  of  the  charac- 
teristics of  a  mud  flow.  The  material  moved  down  the  slopes  while  soft 
and  acted  in  some  measure  as  a  fluid  mixture  of  rock  and  water. 

The  long  body  southwest  of  Mammoth  Mountain  might  be  called  a  rock 
stream.  It  came  from  the  great  cliffs  of  the  Willow  Creek  rhyolite.  The 
rock  mass  that  broke  from  the  cliffs  readily  crumbled  into  small  fragments 
and  flowed  down  the  small  gulley,  much  as  a  large  mass  of  crushed  rock  or 
coal  flows  down  a  chute  of  gentle  incline,  forming  a  crumpled,  humraocky 

surface. 


Economic  c  ons  iderat  ion. 

Considerable  prospecting  has  been  done  in  the  landslides,  notably  east 
of  Deerhorn  Creek.  Evidently  any  bodies  of  ore  found  in  a  landslide  will 
be  small  and  discontinuous  and  the  ledges  in  place  from  which  they  came  will 
be  on  the  slopes  above.   If  the  object  of  the  prospecting  is  to  uncover  a 
vein  in  the  bedrock  beneath  the  slide  rock,  an  estimate  of  the  thickness  of 
the  mantle  of  slide  rock  is  important.  Over  nearly  all  the  area  indicated  as 
landslide  on  the  geologic  map  (Plate  II)  the  tMckness  is  probably  to  be 
measured  in  tens  of  feet  and  over  much  of  it  in  hundreds  of  feet. 

Talus .  <*•**- 

Accumulations  of  rock  debris  at  the  base  of  steep  slopes  from  the 
weathering  of  the  rock  above  cover  considerable  areas  and  obscure  the 
bed  rock  geology  in  many  places.   No  attempt  has  been  made  to  map  these 
consistently,  as  they  are  by  nature  indefinite  bodies.  The  large  talus 
accumulations  above  the  Helson  Tunnel  prevented  a  satisfactory  interpre- 
tation of  the  bed  rock  geology  and  is  indicated  on  the  geologic  map 
(Plate  II)  in  the  same  pattern  as  the  landslides. 

Alluvial  fans. 

.There  the  smaller  streams  and  gulches  reach  the  valleys  of  the  larger 
streams  at  torrential  stages  they  deposit  their  loads  of  sand  and  rock, 
which  accumulate  as  a  more  or  less  conical  or  fan-shaped  body  called  an 
alluvial  fan.  These  fans  which  have  been  separated  in  a  general  way  from 
the  main  alluvium,  are  found  chiefly  in  the  area  of  gentler  topography  near 
the  valley  of  the  Rio  Srande. 


Alluvium. 

The  alluvium  here  includes  the  recent  deposits  of  the  present  streams 
i  excepting  the  alluvial  fans.  Most  of  it  is  ordinary  stream  wash  of  sands 
and  gravels  and  is  but  little  above  the  present  stream  channel.   It  is 
confined  almost  entirely  to  the  lower  courses  of  the  streams  and  the  upper 
glaciated  portions,  as  the  canyons  are  too  sharp  and  the  stream  gradients 
too  steep  for  its  extensive  accumulation  in  the  unglaciated  portions  of  the 
canyons. 

The  body  in  Rat  Creek,  about  a  mile  northwest  of  Bulldog  Mountain 
represents  the  filling  of  a  small  lake  formed  by  the  large  landslide  from 
the  east. 


/3o 


Chapter  IX. 

Structure. 
Introduction. 

The  mountains  for  many  miles  on  all  sides  of  Creede  are  made  up  of 
volcanic  rocks  of  much  the  same  character  and  belonging  to  the  same  general 
volcanic  era.  Their  structure  is  simple  on  the  whole,  and  it  is  character- 
ized by  gentle  tilting  toward  the  Rio  Grande,  and  by  a  few  zones  or  areas 
of  block  faulting.  The  Creede  mining  district  embraces  part  of  such  an  area, 

In  a  number  of  places  the  rocks  near  the  faults  or  between  two  faults 
show  considerable  dips  and  have  evidently  been  tilted  during  the  faulting. 
In  others,  as  at  the  north  end  of  the  Alpha  fault,  the  faults  appear  to 
merge  into  zones  of  steep  dip,  probably  associated  with  more  or  less  minor 
faulting.  Bxcept  in  the  immediate  vicinity  of  the  faults  the  rocks  lie 
nearly  flat  and  probably  dip  gently  toward  the  south. 

Faulting. 

Difficulties  in  detecting  faults. 

Considerable  difficulty  was  experienced  in  locating  and  interpreting 
the  faulting  of  the  district,  chiefly  on  account  of  the  great  and  varying 
thickness  of  nearly  all  the  rock  bodies,  the  lack  of  regular  bedding  planes 
and  horizon  markers,  the  frequent  pinching  out  in  a  very  short  distance  of 
one  or  more  of  the  geologic  units,  the  general  lack  of  a  regular  and  per- 
sistent succession  of  rocks,  the  marked  irregularity  at  the  base  of  a 
large  number  of  the  rock  bodies,  the  cover  of  quaternary  material  on 
critical  areas,  and  in  places  the  difficulty  of  distinguishing  between 
some  of  the  rocks.  The  irregular  character  of  the  faulting  has  added  to 


/3f 


the  difficulty.   A  glance  at  the  geologic  map  (Plate  II)  will  amply  illustra 
most  of  these  points.  However,  a  satisfactory  interpretation  of  nearly  all 
the  important  faulting  is  made  possible  by  the  great  relief,  the  comparative! 
good  natural  exposures,  and  the  considerable  amount  of  prospecting.  Although 
some  points,  such  as  the  northern  extension  of  the  Amethyst  fault,  have  not 
been  settled  beyond  doubt,  the  mapping  as  shown  on  Plate  II  is  believed  to  be 
essentially  correct. 

General  character  of  faulting. 

Most  of  the  major  faults  are  normal  faults  and  strike  a  little  west  of 
north.  The  Amethyst  fault  and  the  Solomon-Ridge  fault  dip  steeply  west, 
the  Bulldog  Mountain  and  the  Alpha  faults  dip  east.  The  block  northeast  of 
the  Corsair  Mine  is  bounded  by  a  nearly  vertical  northwest  fault  and  by  a 
poorly  exposed  southeast  fault.  South  of  the  Commodore  Mine  the  Amethyst 
fault  breaks  up  into  a  number  of  faults  with  varying  strike,  but  their 
general  course  is  considerably  more  to  the  east  than  is  that  of  the  main 
fault.  The  great  Equity  fault  strikes  nearly  neast  and  west  and  dips  steep- 
ly to  the  north;  it  is  the  only  reversed  fault  that  has  been  recognized  in 
the  area  mapped. 

In  general,  brecciation  of  the  walls  adjacent  to  the  faults  is  not 
extensive  and  commonly  the  walls  are  clear  cut  and  slickensided.   Not 
uncommonly,  however,  as  in  the  Amethyst  fault  at  the  Commodore  Mine  and  in 
the  Rid^e  fault  near  the  Rid-e  and  Solomon  Mine,  the  faults  branch  forming 
large  horses,  or  bodies  of  rock  inclosed  between  the  two  branches  of  faults. 

One  of  the  most  striking  characteristics  of  the  faults  is  the  manner 
in  which  they  die  out  along  the  strike.   The  great  Amethyst  fault,  south- 
east of  the  Commodore  Mine,  breaks  up  into  a  number  of  faults,  several  of 


which  have  throws  of  over  a  thousand  feet  and  none  of  which  appears   to 
far  to  the  south  or  east.        To  the  north  of  the  park  Regent  Mine  the  throw 
of  the  Amethyst   fault   is  believed  to  rapidly  decrease  and  is  probably  not 
great   south  of  the  Equity  Mine,  but  here  the  great  Equity  fault   joins  it   and 
beyond  the  junction  it  again  has  a  great   throw  for  some  miles;  farther  north 
it  has  not   been  recognized. 

The  Alpha  and  the  Ridge  faults  pass   into  folds  at   their  north  efctremitiei 
probably  with  considerable  fracturing,  and  are  lost.        S0rae  faults,  as  the 
Equity  at   its  western  extremity,   end  abruptly  against   other  faults.     Sharp 
turns  are  not  uncommon  as  in  the  Amethyst   fault  at  the  Commodore  Mine  and  the 
Alpha  fault  near  the  Xreutzer  Mine, 

Age* 

Nearly  or  quite  all  the  faulting  is  believed  to  have  taken  place  at 
about  the  same  period  in  the  history  of  the  region  and  it  is  probable  that 
the  chief  displacement  occurred  during  a  rather  brief  geologic  period. 
This  period  preceded  metalization.  However,  minor  movements  have  taken 
place  since  the  metalization  as  is  evidenced  by  some  crushing  and  slicken- 
siding  of  the  vein  material  in  nearly  all  the  veins.   The  age  of  all  the 
faults  can  not  be  positively  determined  owing  to  the  limited  distribution 
of  the  later  volcanic  rocks.  But  the  Alpha  fault  cuts  both  the  MacKenzie 
Mountain  quartz  latite  and  the  intrusive  quartz  latite  porphyry  and  no 
fault  is  interrupted  by  later  igneous  rocks.     The  Amethyst  fault  is 
known  to  cut  the  Greede  formation.   It  is,  therefore,  probable  that  at 
least  the  major  faulting  took  place  after  the  extrusion  of  the  youngest 
volcanic  rocks  of  the  area. 


However,  in  the  surrounding  country  a  considerable  thickness  of  volcanic 
rocks,  younger  than  any  in  the  area  here  described,  are  known.  The  MacKenzie 
Mountain  quartz  latite  represents  only  the  base  of  a  considerable  thickness 
of  related  flows  and  breccia  beds  and  at  least  one  later  group  of  rocks  is 
known.   It  is  not  necessary  to  assume  that  the  faulting  is  later  than  these 
younger  volcanic  rocks  and  it  may  have  occurred  in  one  of  the  more  recent 
periods  of  volcanism  whose  products  are  not  preserved,  if  they  ever  occurred, 
in  the  Creede  Special  area. 

The  faulting  took  place  long  before  the  glaciation.  Indeed  since  the 
faulting,  erosion  has  removed  a  thickness  of  rock  measured  in  thousands  of 
feet.  This  is  evident  from  a  consideration  of  the  geologic  imp  (Plate  II). 
Faulting  that  takes  place  rapidly  and  reaches  the  earth's  surface  must  de- 
velop along  the  outcrop  an  escarpment  in  which  the  surface  of  the  up  thrown 
side  of  the  fault  is  raised  relative  to  that  of  the  downthrown  side  by 
an  amount  equal  to  the  throw  of  the  fault.  Although  the  Alpha,  the  Equity, 
the  Amethyst,  and  the  three  faults  east  of  North  Oreede  all  have  throws  of 
over  a  thousand  feet  they  have  no  escarpments  and  they  affect  the  topography 
only  in  so  far  as  they  bring  together  rocks  of  different  hardness.   For 
example,  East  7/illow  Creek  crosses  the  extension  of  the  Amethyst  fault  at  a 
point  where  the  throw  is  about  1500  feet,  yet  there  is  no  change  in  the 
gradient  of  the  stream.  To  accomplish  this  there  must  have  been  removed 
by  erosion  1500  feet^ore  of  rock  on  the  north  side  of  the  faulting  than 
on  the  south  side.  At  the  time  of  the  faulting  the  topography,  therefore, 
must  have  been  very  different  from  the  present  topography  and  the  faulting 
must  have  taken  place  at  the  time  before  the  present  drainage  system  came 
into  existence  or  at  least  when  it  was  very  youthful.  In  this  connection 


it  must  be  remembered  that  a  considerable  thickness  of  volcanic  rocks 
younger  than  any  found  about  Creede,  is  present  in  surrounding  areas  and 
was  probably  once  present  in  this  area. 

A  glance  at  the  geologic  map  (Plate  II)  shows  that  many  of  the  faults 
parallel  the  main  stream,  and  suggests  a  close  relation  between  the  two. 
However,  the  faults  strike  a  little  east  of  south  and  the  natural  course 
of  the  streams  is  directly  toward  the  Rio  Grande  valley,  which  is  southwest. 
There  is  no  evidence  that  the  faults  have  diverted  the  streams  from  their 
normal  courses. 

Comparatively  small  areas,  much  broken  up  by  block  faulting,  chiefly 
of  the  normal  type  but  with  occasional  reverse  faults,  surrounded  for  con- 
siderable distances  by  nearly  flat  rocks  in  which  important  faulting  is 
almost  entirely  absent,  are  common  in  great  volcanic  regions  and  geologic 
relations  often  indicate  that  the  faulting  was  more  or  less  closely  re- 
lated to  the  igneous  or  volcanic  activity  of  the  region.  The  faulting 
about  Creede  is  of  this  character  and  is  believed  to  have  taken  place  in 
late  in  the  period  of  igneous  activity  and  before  the  existing  mountains  and 
canyons  of  the  region  were  developed. 

Mineralization  along  faults. 

Nearly  all  the  faults  of  the  region  show  some  mineralization  and  nearly 
all  the  ore  produced  has  come  from  veins  along  faults.  The  chief  production 
has  come  from  the  Amethyst  vein,  which  is  along  the  major  fault  of  the  area. 
The  ore  produced  at  Sunnyside  Camp  came  from  the  vein  along  the  Alpha  fault ; 
the  S0lomon,  Ridge,  and  Holy  Moses  mines  are  along  the  Solomon-Ridge  fault, 


and  the  Equity  Mine  is  along  the  great  Equity  fault.  Of  the  five  major 
faults  or  fault  systems  the  Bulldog  Mountain  fault  is  the  only  one  that 
has  produced  no  ore. 

The  chief  production  of  the  region  has  come  from  the  Amethyst  fault, 
between  the  south  end  line  of  the  Bachelor  and  the  north  end  line  of  the 
Park  Regent  claims.  North  of  the  Park  Regent  the  vein  has  not  been  located 
and  in  the  network  of  faults  into  which  the  Amethyst  breaks  southeast  of 
the  Commodore  Mine,  no  large  ore  bodies  have  as  yet  been  developed. 

The  Amethyst  fault  system. 

The  Amethyst  fault  system  is  the  most  prominent  structural  feature 
of  the  region  and  it  gains  additional  importance  from  the  fact  that  the 
productive  Amethyst  vein  is  along  it.   It  is  here,  somewhat  arbitrarily, 
taken  to  embrace  the  main  fault  between  the  Bachelor  and  Park  Regent 
mines,  the  network  of  faults  extending  south  and  southeast  of  the  Bachelor 
Mine,  to  and  beyond  Lianmoth  Mountain,  and  the  poorly  exposed  system  of 
faulting  from  the  Captive  Inca  shaft  to  and  beyond  the  Equity  Mine. 
Strictly  the  Equity  fault  might  be  considered  a  part  of  this  system  but 
it  is  discussed  separately  herein. 

North  of  the  Commodore  Lline  the  Amethyst  fault  is  a  clean  cut,  nearly 
straight  fault  with  a  strike  of  about  N.  23°  'V.  and  a  dip  of  about  50° 
to  70°  y.   The  western  or  hanging  wall  at  the  surface  is  made  of  the 
upper  member  of  the  Creede  formation  in  the  southern  part  and  of  7/indy 
Gulch  rhyolite  breccia  in  the  northern  part;  only  a  few  hundred  feet  below 
the  surface  in  the  underground  workings  the  hanging  wall  is  the  Campbell 
Mountain  rhyolite  and  this  continues  to  the  Nelson  Tunnel  level.  The 


throw  must,  therefore,  be  at  least  1400  feet,  in  that  part  of  the  fault 
which  is  north  of  the  branch  north  of  the  Commodore  Mine  there  are  no 
sharp  changes  in  direction  of  the  fault  plane,  and  no  branching  faults 
of  any  considerable  size  have  been  recognized,  with  the  exception  of  a 
po  sible  fault  of  unknown  throw  in  the  hanging  wall  just  south  of  the 
Last  Chance  Mine.   In  the  Last  Chance  and  New  Y^rk  mines  there  is  a 
great  amount  of  fissuring  and  crushing  of  the  hanging  wall  rocks.  Through- 
out this  distance  exposures  are  fairly  good.  «^Ii4^I^2U^-^¥aM^8po^lJte^jx, 

~£&es^»*aifeie«M«*«Le«it«if^^ 

~siiafjt«--tt£Hr?Tk_#ri^ 

North  of  the   south  end  line  of  the  Commodore  claim  the  fault  branches 
toward  the   south.     The  details  of  this  faulting  are  shown  on  Plate  n  nrnd 

•UST.       The  main  branch  is  on  the  west   and  has  a  throw 


of  at  least  700  feet,  while  the  eastern  branch  has  a  throw  of  at  least 
100  feet,  making  the  combined  throw  at  least  800  feet.  It  is  probably 
much  more, 

The  two  branches  come  together  again  near  the  south  end  of  the 
Commodore  claim.  To  the  southeast  exposures  are  poor,  but  there  must  be 
a  great  fault,  striking  nearly  southeast  and  well  exposed  at  only  one 
small  outcrop  about  half  way  between  the  Nelson  Tunnel  and  the  portal  of 
the  adit  which  on  the  map  is  designated  as  Commodore  Mine.  Beyond  this 
there  are  no  surface  IX  exposures  west  of  \7est  Willow  Creek.  Farther  to 
the  south  and  east  there  is  a  network  of  related  faults  and  no  one  of 
them  alone  can  be  considered  the  continuation  of  the  Amethyst  fault. 


A  great  fault  with  a  throw  of  about  1500  feet  crosses  the  nose  of  the 
ridge  between  the  forks  of  71  How  Creek.  It  strikes  about  s.  60°  B.  and 
has  a  steep  dip.   It  is  indicated  chiefly  by  the  manner  in  which  the 
Campbell  Mountain  rhyolite  abuts  against  the  Willow  Creek  rhyolite,  but  it 
is  poorly  exposed  in  one  tunnel  -  on  the  east  side  of  the  ridge.  This 
fault  continues  with  about  the  same  course  and  crosses  the  ridge  south  of 
ffiammoth  Mountain.  In  some  of  the  tunnels  where  it  is  best  exposed  it 
shows  considerable  brecciation  in  the  hanging  wall  and  some  mineralization. 
Where  it  separates  the  Campbell  Mountain  and  Willow  Creek  rhyolites  its 
position  is  appro ximately  shown  by  the  abrupt  change  from  the  cliffs  of  the 
latter  rhyolite  to  gentle  talus-covered  slopes  of  the  Campbell  Mountain 
rhyolite.  S0uthwest  of  Mammoth  Mountain,  however,  it  joins  in  an  acute 
angle  with  a  fault  which  lies  south  of  it  and  which  has  a  large  throw  in 

the  opposite  direction.  To  the  east  of  this  junction  the  fault  continues 

with 
to  andbeyond  the  east  boundary  of  the  area  included  in  the  map  but /a  throw 

that  is  probably  not  more  than  two  hundred  feet. 

The  southernmost  of  these  two  faults  is  nowhere  well  exposed,  but  the 
rocks  are  well  exposed  on  both  sides  of  the  fault  and  its  position  is  in- 
dicated by  a  change  form  the  cliff-like  outcrops  of  the  Willow  Creek  rhyolite 
to  the  south,  to  the  gentler  talus-covered  slopes  of  the  Campbell  Mountain 
rhyolite  to  the  north.   It  strikes  about  S.  70°  E  and  has  a  throw  estimated 
at  several  hundred  feet  with  the  downthrow  on  the  north  side.  Its  dip  is 
not  known  but  it  is  probably  nearly  vertical.  This  and  the  fault  to  the 
north  leave  a  narrow  sharp-pointed  wedge  of  the  Campbell  Mountain  rhyolite 
surrounded  by  Willow  Creek  rhyolite.  This  triangular  block  is  bounded  on 
the  west  by  a  third  great  fault  with  downthrow  on  the  west  side. 


The  latter  fault  has  a  strike  a  little  east  of  south,  it  is  nowhere 
well  exposed  but  its  presence  is  shown  by  the  Campbell  fountain  rhyolite 
abutting  against  the  pillow  Greek  rhyolite  east  of  East  '.Villow  Greek  and 
by  the  presence  of  Campbell  Mountain  rhyolite  forming  gentle  slopes  at  the 
base  of  great  cliffs  of  'Pillow  Creek  rhyolite.   Its  position  could  not  be 
accurately  located  especially  south  of  Bast  Billow  Creek  as  talus  covers 
much  of  this  area.   VThere  it  crosses  the  creek  it  has  a  throw  of  about 
a  thousand  feet  and  beyond  the  point  where  the  nearly  east -west  fault 
joins  it  the  throw  is  increased  to  about  1500  feet.  To  the  south,  after 
being  joined  by  a  number  of  small  cross  faults,  it  is  covered  by  landslide 
and  talus  before  reaching  the  Creede  formation.  It  is  believed  that  this 
fault  rapidly  decreases  in  throw  to  the  south,  partly  by  tilting  of  the 
fault  blocks,  partly  b  y  the  cross  faulting,  but  it  is  not  likely  that 
it  dies  out  before  reaching  the  Creede  formation,  although  it  was  not 
located  in  that  formation. 

To  the  west  of  this  north-south  fault  four  cross  faults  with  throws 
up  to  250  feet  or  more  are  shown  on  the  geologic  map  (Plate  II).  These  were 
located  largely  by  the  displacement  of  the  base  of  the  Campbell  Mountain 
rhyolite. 

The  three  faults  on  the  opposite  side  of  Willow  Creek  are  all  of 
small  throw,  That  just  north  of  Windy  Gulch  forming  the  south  boundary  of 
the  Campbell  Mountain  rhyolite  is  shown  in  a  few  prospect  pits.  It  has  a 
throw  of  less  than  a  hundred  feet.  The  other  two  faults  are  indicated 
chiefly  by  slight  displacement  of  the  intrusive  rhyolite. 

Just  west  of  West  Willow  Creek,  from  the  main  Amethyst  fault  to  the 
south  there  is  probably  a  fault  of  considerable  throw.  This  fault  is  indi- 
cated by  the  fact  that  the  base  of  the  Campbell  Mountain  rhyolite  on  the 


A?  7 


east  side  of  West  Willow  Creek  is  nearly  flat  and  at  an  elevation  of  about 
9050  feet  while  on  the  west  side  of  the  Greek,  less  than  a  quarter  of  a  mile 
away,  the  base  of  the  Campbell  Mountain  rhyolite  is  at  an  elevation  of  9750 
feet.  Moreover,  a  few  hundred  feet  in  the  Nelson  M«ra»fea*  tunnel  the  Campbell 
Mountain  rhyolite  forms  the  hanging  wall  of  the  vein  although  on  the  hill  to 
the  west  its  base  is  about  400  feet  higher. 

The  northern  extension  of  the  Amethyst  fault  beyond  the  Park  Resent  Mine 
can  not  be  so  positively  determined  owing  to  the  lack  of  exposures  at  critical 
places  over  much  of  the  area  and  to  some  uncertainties  in  the  interpretation 

of  the  bed  rock  geology  west  of  Deerhorn  Creek,  prom  the  Happy  Thought  shaft 

•*•• 

to  Deerhorn  Creek  the  glacial  moraine  covers  the  bed  rock  nearly  everywhere 
and  for  half  a  mile  beyond  this,  landslide  covers  the  line  of  the  fault. 
Beyond  the  Park  Regent  Mine  there  are  few  prospect  shafts  that  show  bedrock 
and  the  interpretation  of  this  area  is  based  on  data  secured  from  these 
and  from  a  simple  outcrop. 

A  short  distance  north  of  the  Park  Regent  shaft  numerous  surface 
prospects  and  the  underground  workings  show  that  the  fault  turns  rather 
sharply  to  the  east  and  has  a  nearly  north-south  strike.  This  strike  prob- 
ably does  not  continue  far  because  there  is  no  place  on  Nelson  Mountain  or 
to  the  west  as  far  as  the  main  branch  of  Deerhorn  Creek,  where  a  fault  of 
any  considerable  throw  could  pass.   Nelson  Mountain  itself  shows  good  ex- 
posures and  is  made  up  of  rather  regular  alternating  flows  and  tuff  beds; 
a  fault  of  even  small  throw  could  easily  be  recognized.  T0  the  west  of 
Nelson  Mountain,  at  an  elevation  of  11500  feet  and  near  the  north  border 
of  the  area  included  in  the  map,  is  an  abrupt  change  in  the  bedrock,  and 
a  large  area  of  the  Equity  quartz  latite  is  exposed.  This  is  not  found 


to  the  southeast.  This  quartz  latite  is  separated  from  the  other  rocks  of 
the  lower  division  of  the  Potosi  to  the  southeast  by  a  considerable  land- 
slide, and  its  relations  here  are  not  entirely  clear.  However,  it  is  a  part 
of  a  great  flow,  or  perhaps  of  several  great  flows,  which  extend  northward 
to  the  Equity  Mine  and  here  clearly  overlie  the  Campbell  Mountain  rhyolite. 
T"  the  north  and  northwest  of  Nelson  Mountain  this  quartz  latite  clearly 
underlies  the  tuffs  and  flows  that  form  the  upper  slopes  of  Nelson  Mountain 
and  if  the  two  are  separated  by  a  fault  on  the  west  slope  of  Nelson  .Mountain 
the  fault  can  have  only  a  small  throw.  The  down  throw  side,  moreover,  would 
be  on  the  east  instead  of  on  the  west  as  in  the  Amethyst  fault.   A  fault 
might  pass  into  this  thick  quartz  latite  series  but  no  evidence  of  such  a 
fault  was  found  and  a  fault  with  a  throw  comparable  to  that  of  the  Amethyst 
fault  should  bring  down  some  of  the  higher  rocks  to  the  west  or  otherwise 
indicate  its  presence. 

There  is  some  evidence  as  to  the  position  of  the  Amethyst  fault  in  this 
moraine-covered  area.  In  the  bed  of  Test  Willow  Creek,  just  west  of  the 
point  where  the  road  crosses  the  creek  is  a  fair  outcrop  of  the  andesite 
and  a  few  hundred  yards  to  the  south  are  some  prospects  that  bring  up  the 
tuff  and  the  andesite.   This  area  is,  therefore,  similar  to  the  isolated 
area  of  good  exposures  just  west  of  the  mouth  of  Deerhorn  Creek  and  belong 
to  the  upper  division  of  the  Potosi.  The  shallow  shaft  shown  on  the  map 
about  two  hundred  yards  southeast  of  this  road  crossing  brings  up  only 
altered  rhyolite  of  a  type  that  can  be  identified  with  reasonable  certainty 
as  belonging  to  the  mi  low  Creek  rhyolite.  The  main  fault  can,  therefore, 
be  placed  with  reasonable  assurrance  between  the  road  and  this  shaft. 


Farther  north,  beyond  Deerhorn  Greek,  the  only  probable  position  for  a 
fault  is  between  the  rocks  of  the  upper  and  lower  divisions  of  the  potosi 
volcanic  series.  The  contact  is  nowhere  well  shown  in  this  area,  but  more 
or  less  alteration  and  mineralization  along  this  line  indicate  a  fault  as 
does  also  the-  nearly  straight  contact  with  the  rocks  of  the  upper  division 
of  the  Potosi  series  below  steep  slopes  of  rocks  of  the  lower  division. 
The  latter  is  not  in  itself  conclusive  evidence  of  a  fault  as  similar  rela- 
tions elsewhere  are  due  to  the  very  rugged  surface  over  which  the  rocks 
of  the  upper  division  of  the  Potosi  were  extruded.  However,  a  fault  of 
considerable  throw  is  believed  to  separate  these  two  divisions  of  the 
Potosi  and  is  so  shown  on  the  geologic  map  (Plate  II). 

This  fault  probably  forks  at  about  the  crest  of  the  ridge;  one  branch 
separates  the  rocks  of  the  upper  and  lower  divisions  of  the  Potosi  while 
the  other  passes  into  the  Equity  latite.  This  latter  fault  is  indicated 
largely  from  the  relations  of  the  rocks  near  the  bed  of  tfest  Willow  Creek 
just  below  the  Equity  Mine.  The  rocks  of  this  area  are  all  of  the  lower 
division  of  the  Potosi.  In  the  creek  bed  there  are  good  exposures  of  the 
Equity  latite,  while  on  the  steep  slopes  to  the  east  the  base  of  this 
formation  is  about  a  hundred  feet  above  the  creek  bed.  This  continues 
to  the  Equity  fault  which  ends  against  the  north-south  fault.  North  of  the 
Equity  fault  the  throw  of  the  north-south  fault  has  greatly  increased.   It 
brings  Willow  Creek  rhyolite  against  Equity  latite.   It  continues  Just  east 
of  the  creek  bed  for  a  mile  or  more  to  the  north  but  it  probably  dies  out 
rapidly  in  that  direction  as  it  could  not  be  located  on  the  divide  at  the 
head  of  the  creek.   Its  position  is  shown  by  an  inconspicuous  series  of 


flats  and  sags  along  its  line  which  Are  no  doubt  erosional  features  and  are  due 
to  the  soft  nature  of  the  rhyolite  to  the  east  as  compared  with  the  latite  to 
the  west. 

The  displacement  of  the  Amethyst  fault,  or  of  the  fault  zone  of  Test 
pillow  Greek  near  Deerhorn  Greek  is  probably  not  great  since  the  base  of  the 
Nelson  Mountain  quartz  latite  is  at  an  elevation  of  about  11600  feet  on  the  wes 
side  of  the  valley  and  on  Nelson  Mountain;  only  a  mile  and  three  quarters  away 
it  is  at  11900  feet,  a  difference  of  only  three  hundred  feet.  The  base  of  this 
flow  is  fairly  level  and  regular  on  both  sides  of  the  creek. 

Equity  fault. 

The  Equity  fault  is  one  of  the  few  structural  features  that  stand  out 
clearly  enough  to  be  correctly  interpreted  from  a  hurried  examination. 
South  of  the  fault  the  Equity  latite  gives  the  rugged  outcrops  and  broken 
cliffs  of  a  hard  resistant  rock.  Along  the  fault  line  which  runs  straight 
up  the  hill  over  very  steep  slopes  for  over  a  thousand  feet,  this  type  of 
outcrop  gives  place  abruptly  to  steep  grass-covered  or  talus-covered 
slopes  of  the  '.?illow  Creek  and  Mammoth  Mountain  rhyolites.  A  zone  of  white 

and  iron-stained  rock  follows  the  fault  plane.  The  fault  is  shown  in 

/v 
Plate -£*¥-,  which  is  a  photograph  taken  from  a  point  on  the  road  opposite 

the  Equity  tunnel.  The  fault  strikes  a  little  south  of  east,  dips  steeply 
to  the  north  and  is  reversed,   in  the  area  included  in  this  report  it  is 
entirely  in  rocks  of  the  lower  division  of  the  Potosi.  The  south  or  foot 
wall  is  of  Campbell  Mountain  rhyolite  near  the  creek  bed  but  in  great 
part  it  is  of  Equity  latite.  The  north  wall  is  of  billow  Creek  rhyolite 
in  the  lower  slopes  and  Campbell  Mountain  rhyolite  on  the  upper  slopes. 
A  quarter  of  a  mile  north  of  the  place  ^here  the  fault  crosses  the  ridge 


between  ^ffest  Willow  and  Deerhorn  creeks  and  only  a  hundred  feet   or  so  higher 
in  elevation,    the  Equity  latite  overlies  the  Campbell  Mountain  rhyolite.     The 
throw  of  the  fault   is  about  1200  feet  as  measured  by  the  displacement   of  the 
base  of  the  Equity  latite. 

The  Equity  fault  ends   on  the  west  against   the  Amethyst  fault,   just  e  ast 
of  West  Willow  Greek.     To  the  east   it   crosses  Deerhorn  Creek  ani  passes  into 
the  drainage  area  of  East  7/illow  Creek  but  with  a  much  diminished  throw. 

There  is  considerable  brecciation  and  mineralization  along  this  fault 
and  in  the  Equity  Mine,   from  which  the  fault   is  named,  a  vein  along  it  is 
worked. 

Fault  block  of  Maramoth  Mountain. 

On  Mammoth  Mountain  are   three  faults  that   may  be  closely  related  to 
the  Amethyst   system.     They  form  a  part   of  the  boundary  of  the  body  of 
Campbell  Mountain  rhyolite.      (See  Plate   II).     The  East-West  fault   on  the 
north  is  probably  the  largest   of  these  faults  and  has  a  throw  estimated 
at  about   five  hundred  feet.      It    is  well  exposed  only  in  the  cliffs  on  the 
west   slope  of  the  mountain  where   it   is  nearly  vertical.        It   separates  the 
Willow  Creek  rhyolite  from  the  Campbell  Mountain  rhyolite  and  to  the  east 
from  the  Llammoth  Mountain  rhyolite. 

The  fault  on  the  west  of  the  Canpbell  Mountain  rhyolite  is  well  ex- 
posed in  the  cliffs  at   its  northern  part.     It  separates  the  Willow  Creek 
rhyolite  on  the  west  from  the  Campbell  Mountain  rhyolite.      Its  throw  is 
at   least  a  hundred  feet   and  is  probably  not  much  greater. 


The  third  fault  of  this  group  separates  the  rhyolites  of  the  lower 
division  of  the  Potosi  from  the  Mammoth  Mountain  rhyolite  and  runs  into  the 
east-west  fault.   This  fault  is  somewhat  uncertain  as  the  only  evidence  of 
its  presence  is  the  form  of  the  contact  and  this  may  be  accounted  for  as  due 
to  the  great  irregularity  at  the  base  of  the  upper  division  of  the  Potosi. 
The  throw  of  this  fault  is  uncertain  but  it  is  probably  considerable. 

Bulldog  Mountain  fault. 

",7est  of  Windy  G-ulch  is  a  fault  which  strikes  a  little  west  of  north  and 
dips  about  50  degrees  to  the  east.  On  the  east  slope  of  Bulldog  Mountain  it 
displaces  the  base  of  the  andesite  about  two  hundred  feet.  To  the  south 
it  plays  out  or  is  lost  in  the  poorly  exposed  ^indy  (lulch  rhyolite  breccia, 
while  to  the  north  it  can  be  followed  more  or  less  continuously  to  the  Rat 
Creek  road.  Its  throw  decreases  to  the  north  and  it  disappears  in  the  tuff. 

There  has  been  some  prospecting  along  this  fault  but  it  shows  less  min- 
eralization than  many  of  the  other  faults  of  the  region. 

Solomon-Ridge  fault. 

The  S0lomon-Ridge  fault  is  west  of  East  Willow  Creek.   It  has  a  small 
throw  but  is  important  because  along  it  are  the  veins  of  the  Solomon,  Ridge, 
and  Holy  Moses  mines.   It  strikes  a  few  degrees  west  of  north  and  dips  steep- 
ly to  the  west.  The  downthrow  side  is  on  the  west  and  the  displacement  is 
probably  only  a  few  hundred  feet.  To  the  south  it  has  been  recognized  only 
a  few  hundred  yards  south  of  the  Solomon  tunnel.  Near  the  Solomon  tunnel 
it  branches  but  the  two  branches  come  together  again  north  of  the  Ridge  Mine 
It  was  followed  northward  to  a  point  nearly  a  mile  north  of  the  Holy  Moses 


!  Mine  and  is  lost  on  the  slopes  west  of  Phoenix  Park  in  an  area  where  exposures 
are  very  poor  and  interpretation  of  the  geology  somewhat  uncertain,  in  this 
area  there  is  some  fracturing  and  mineralization  which  is  probably  related  to 
the  Ridge  fault. 

Alpha  Fault. 

The  Alpha  Fault  east  of  Miners  Creek  is  a  characteristically  crooked 
fault.  Along  it  are  the  Alpha,  Corsair,  and  Kreutzer  mines,  and  north  of 
the  sharp  turn  at  the  Kreutzer  Iline  it  has  been  prospected  nearly  to  the 
crest  of  MacXenzie  Mountain, 

In  its  southern  part  between  the  Alpha  and  Kreutzer  mines  its  average 
atrike  is  about  N.  31°  7.  and  it  dips  at  from  54° to  65  degrees  to  the  east. 
The  hanging  wall  is  commonly  much  fractured.  The  fault  throws  down  the 
Campbell  Mountain  rhyolite  on  the  east  against  Willow  Creek  rhyolite.  The 
displacement  along  the  fault  in  this  southern  part  is  not  known  but  it  is 
believed  not  to  be  great.  A  narrow,  much  fractured  dike  of  quartz  latite 
porphyry  forms  the  southwest  wall  in  parts  of  the  Alpha  Mine;  a  little  to 
the  north  a  similar  dike  forms  the  northeast  wall  and  it  is  probably  the 
same  dike  indicating  a  considerable  movement  of  the  east  wall  to  the  north, 
A  little  farther  north  another  dike  of  the  same  rock  forms  the  east  wall  of 
the  fault  for  a  considerable  distance.  The  relation  of  these  dikes  to  the 
faulting  is  not  entirely  clear  but  some,  possibly  all,  of  the  faulting  is 
later  than  the  dikes. 


A  few  hundred  yards  south  of  the  Kreutzer  Mine  a  very  poorly  exposed 
cross  fault  joins  the  main  fault  from  the  east.   Its  exact  relations  are 
uncertain  as  it  has  not  precept ibly  displaced  the  quartz  latite  dike  al- 
though it  is  believed  to  have  crossed  it.  If  the  quartz  latite  porphyry 
dike  formed  a  nearly  continuous  tain  layer  on  the  hanging  wall  of  the  main 
fault,  as  it  appears  to  do  on  the  surface,  a  moderate  displacement  by  a 
cross  fault  might  not  be  perceptible  in  a  poorly  exposed  area.   The  throw 
of  this  fault  is  not  known  but  it  is  believed  that  the  north  side  has  been 
dropped  several  hundred  feet  near  the  Alpha  fault  and  that  this  displacement 
rapidly  decreases  to  the  east.  It  therefore  gives  the  Alpha  fault  an  in- 
crease throw  to  the  north. 

At  the  Kreutzer  Mine  the  Alpha  fault  makes  a  sharp  turn  to  the  north 
and  beyond  it  has  an  average  course  of  a  few  degress  west  of  north;  it 
dips  steeply  to  the  east. 

It  is  a  remarkable  fact  that  Just  north  of  the  Kreutzer  Mine  the  top 
of  the  Campbell  Mountain  rhyolite  is  dropped  over  a  thousand  feet  by  this 
fault,  yet  but  a  mile  to  the  north,  near  the  crest  of  the  MacKenzie  Mountain 
Ridge,  the  andesite  is  dropped  but  little.  This  great  change  in  the  amount 
of  displacement  along  the  fault  is  explained  by  the  dip  of  the  rocks  lying 
to  the  east  of  the  fault.  Few  of  the  rocks  of  this  area  are  of  such  char- 
acter as  to  afford  opportunity  for  even  rough  estimates  of  dips  and  strikes 
but  the  relations  are  brought  out  clearly  by  mapping  the  various  rock  bodies 
(see  Plate  II).  It  is  evident  that  the  flows  to  the  west  of  the  fault  are 
nearly  flat  or  dip  gently  to  the  south,  while  those  to  the  east,  on  the  ridge 
of  MacXenzie  Mountain,  dip  at  a  considerable  angle  to  the  southeast  but  in 


going  south  their  dip  swings  to  the  south  and  in  the  wedge  included  in  the 
angle  of  the  main  fault,  and  north  of  the  cross  fault  (see  Plate  II)  they 
apparently  dip  to  the  southwest. 

North  of  MacXenzie  Mountain  the  fault  passes  into  an  area  covered  by 
Quaternary  rocks  and  is  lost.  For  a  mile  beyond,  however,  the  rocks  show 
considerable  tilting  which  is  no  doubt  related  to  the  faulting.  This 
structure  is  best  seen  in  the  tridymite  latite  in  which  the  good  flow  lines 
are  normally  nearly  or  quite  flat.   In  this  area  these  flow  lines  are 
especially  well  shown  in  the  isolated  body  of  tridymite  latite  which  is 
west  of  the  earlier  glacial  moraine  (see  plate  II).  This  entire  body 
dips  to  the  east  at  from  35  to  40  degrees  on  the  upper  slopes  and  at  some- 
what smaller  angles  on  the  lower  slopes.   Indeed  this  slope  is  nearly 
a  dip  slope  and  is  probably  at  about  the  top  of  the  tridymite  latite  since 
the  andesite  appears  on  the  northwest  corner  of  this  outcrop  with  its  base 
at  an  elevation  of  11250  feet  while  less  than  half  a  mile  to  the  east  it 
is  at  an  elevation  of  10450  feet.  The  MacKenzie  Mountain  quartz  latite  is 
probably  also  affected  by  these  structures.   Its  base  is  nearly  everywhere 
covered  by  landslide  but  over  nearly  the  whole  of  this  area  the  top  of  the 
landslide  laps  but  little  upon  the  latite  as  is  indicated  by  the  exposure 
of  the  glassy  base  of  the  latite  at  very  numerous  places.  No  faulting  was 
observed  in  this  area  of  steep  dip  but  minor  faulting  is  probable;  indeed, 
without  fracturing,  it  is  difficult  to  conceive  how  these  sharp  changes  in 
structure  could  take  place  in  brittle  rock  which  was  under  only  a  moderate  loa 

This  zone  of  steep  dips  has  not  been  recognized  as  far  north  as  the 
later  glacial  moraines.  It  is  confined  to  a  narrow  strip,  less  than  half 
a  mile  wide  and  about  two  miibes  long. 


7/5- 


About  a  mile  or  less  to  the  east  of  this  on  the  east  side  of  Hat  Creek 
is  an  area  of  rocks  which  probably  have  considerable  dips  to  the  west 
although  the  structures  are  not  clearly  shown  and  are  indicated  chiefly  by 
the  form  of  contacts  that  are  nowhere  regular,  and  by  the  great  vertical 
extent  covered  by  the  7indy  Gulch  rhyolite  breccia.    These  two  belts 
would  make  a  canoe-shaped  syncline  in  Hat  Greek  basin,  with  the  point  of 
the  canoe  at  about  the  south  end  of  the  upper  glacial  moraine.  The  contin- 
uation of  this  zone  of  complex  structure  to  the  north  may  account  for  the 
irregularities  in  the  relations  of  the  andesite  and  tridymite  latite  in 
Hat  Greek,  just  south  of  the  northwest  corner  of  the  area  included  on  the 
map.  Exposures  are  poor  in  this  area  and  no  satisfactory  interpretation 
could  be  made.  TO  the  south  this  structure  probably  extends  to  the  fault- 
ing east  of  Corsair  Mine.  This  small  structural  basin  is  bounded  to  a 
considerable  extent  by  faults  of  large  throw  and  is  probably  associated 
with  much  minor  faulting* 

Structure  and  faulting  northeast  of  Sunnyside. 

Northeast  of  Sunnyside,  as  shown  by  the  geologic  map  (Plate  II),  there 
is  an  area  of  block  faulting,  with  tilted  blocks.  Poor  exposures,  the 
irregular  erosional  surfaces  separating  each  of  the  members,  the  decomposition 
of  the  rocks,  and  their  lack  of  development  in  their  most  characteristic 
forms,  introduce  some  uncertainty  into  the  mapping  and  interpretation  of  the 
structure. 


The  rocks  of  this  area  are  the  '.Tillow  Creek  and  Campbell  Mountain  rhyo- 
lites,  the  intrusive  rhyolite,  the  hornblende-quartz  latite,  the  Tindy  Oulch 
rhyolite  breccia,  the  tridyraite  latite,  and  the  various  members  of  the 
Creede  formation.   Here  the  Campbell  Mountain  rhyolite  resembles  both  the 
underlying  '/Villow  Creek  rhyolite  and  the  T7^ndy  Gulch  rhyolite  breccia  anl 
separation  is  difficult.  The  hornblende-quartz  latite  is  poorly  exposed 
and  is  not  altogether  characteristic;  the  tridymite  latite  is  fairly 
characteristic,  and  the  rocks  of  the  Creede  formation,  except  for  sub- 
ordinate tuff  in  the  hornblende-quartz  latite,  are  easily  distinguished. 

A  nearly  vertical  fault,  whichstrikes  about  N.  40°  E.  forms  the  north- 
west boundary  of  this  area.  On  the  west  side  of  Rat  Creek  it  is  poorly 
exposed  and  is  somewhat  uncertain,  but  from  Rat  Creek  to  the  northeast  it 
is  easily  followed.   Just  east  of  Rat  Creek  for  several  hundred  yards 
its  line  is  marked  by  low  cliffs  of  the  Billow  Creek  rhyolite  facing  to 
the  southeast.   (Plate  X'Efc).  The  slopes  below  only  a  few  feet  northeast 
of  these  cliffs,  show  fair  exposures  of  the  tridymite  latite  which  is 
here  soft  and  weathers  into  low  pinnacles.  The  flow  banding  of  this  is 
cut  off  sharply  by  the  cliff  of  rhyolite  and  prospects  along  the  fault 
expose  the  actual  contact  at  a  number  of  places.  There  is  little  miner- 
alization. The  throw  of  the  fault  can  not  be  estimated,  but  it  is  consid- 
erable near  Rat  Creek,  and  it  is  believed  to  decrease  rapidly  to  the 
northeast  as  the  latite  dips  to  the  southwest,  ffo  trace  of  this  fault  was 
seen  aouth  of  the  Corsair  Mine  and  it  probably  goes  under  the  alluvium  and 
is  lost  in  the  alluvium  and  tuff.   It  may  join  the  Alpha  fault  in  which  case 
it  would  increase  the  throw  of  that  fault  to  the  south.  To  the  northeast  it 
can  be  followed  nearly  to  the  crest  of  the  rid^e  and  it  must  quickly  die  out 

or  join  a  fault  running  southeast. 


The  southeast  contact  of  this  narrow  strip  of  tridymite  latite  may  also 
be  a  fault.  East  of  Rat  Creek  the  nearly  straight  contact  cutting  across 
the  topography  suggests  a  fault  although  normal  contacts  at  the  base  of  the 
lake  beds  are  of  this  character  in  other  places.  The  contact  is  indicated 
as  normal  on  the  map  as  the  facts  can  be  interpreted  as  well  without  assuming 
a  fault  to  be  present. 

Five  other  faults  all  of  small  throw  have  been  mapped  in  this  area. 
They  have  throws  of  from  fifty  to  a  few  hundred  feet  and  probably  continue 
for  no  great  distance  along  their  strikes.  The  east-west  fault  on  the  north 
boundary  of  the  tridymite  latite  strip  is  nowhere  exposed  but  its  presence 
is  reasonably  certain  from  the  form  of  this  contact.  The  way  in  which  the 
Creede  formation  is  displaced  by  the  east  extension  of  this  fault  indicates 
that  the  north  block  moved  to  the  west.  The  northeast  boundary  of  the 
tridymite  latite  body  also  is  believed  to  be  a  fault  on  account  of  the 
form  of  this  contact.  This  fault  probably  continues  to  the  northwest  and 
joins  the  main  fault  of  this  block.  The  two  small  faults  that  cut  the 
hornblende-quartz  latite  are  well  exposed  to  prospect  pits.  The  supposed 
fault  bounding  this  latite  on  the  northeast  is  nowhere  exposed  and  this  may 
be  a  normal  contact. 

Minor  faults. 

in  addition  to  the  faults  already  described  are  a  large  number  of  faults 
and  slips  of  uncertain  but  probably  small  throw,  In  some  the  throw  may  be  a 
hundred  feet  or  even  more  and  they  could  be  consistently  mapped  in  an  area 
of  comparatively  regularly  bedded  rock,  but  not  without  great  difficulty  and 
uncertainty  in  this  area.  Only  a  few  of  these  are  indica  ted  on  the  geologic 


map, 


ILLUSTRATIONS. 


PLATE  I.  Topographic  map  of  the  Creede  Special  area  ....    Pocket 

II*  Geologic  map  of  the  ureede  Special  area  .  *  .  .  .    Pocket 

III.  Creede  .....  

IV.  Photomicrographs  of  Willow  Creek  rhyolite  .  .  .  * 

V.  Photomicrographs  of  Willow  Creek  rhyolite  .... 

VI.  Photomicrographs  of  Willow  Creek  rhyolite  .... 

VII.  Willow  Creek  Canyon  above  Creede  ......... 

VIII.  Hugged  cliffs  of  Willow  Creek  rhyolite 

IX.  Equity  fault  

X.  Fault  east  of  Sunnyside • 


T La.  ire. 


, 

*%i   ;'  ? 
,  1  -• 


III.       Creede 


Frontispiece 


1 


Photomicrographs  of  villow  Creek  rhyolite.  Plane  polarized  light 
with  inclined  illumination  to  bring  out  relief.  Show  two  of  coarse 
"bands.  Magnified  10  diameters. 


V 


Photomicrographs  of  v/iilow  Greek  rhyolite.  Plane  polarized  light 
with  inclined  illumination  to  bring  out  relief.  Across  one  of  the 
coarse  bands,  ilagnified  40  diameters. 


".  •'      '  •• 


Photomicrograph  of  'Villow  ^reek  rhyolite.  Plane  polarized  light. 
Inclined  illumination  to  bring  out  relief.  A  part  of  one  of  the 
broad  bands,  ilagnified  40  diameters. 


\7illow  Creel:  Canyon  above  Creede.  Shows  cliffs  of  Villow  Creek  rhyolite 
with  overlying  gentler  slopes  of  Campbell  Mountain  rhyolite. 


sr     -  •;    .    •.    '    iaH».yi.i  ML 


fill.  Rugged  cliffs  of  7/illow  Creek  rhyolite.  Above  forks  of  '.'/illow  Creek. 


Equity  fault,  from  road  across  from  Equity. mine. 


c 


X.       Fault  east  of  Sunny-side,   from  Sunnyside< 


I1UI1 


722075 


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